Optical recording medium and optical recording apparatus

ABSTRACT

There is provided an optical recording medium comprising a recording layer containing a charge-generating material capable of generating a first electric charge and a second electric charge by beam irradiation, the second electric charge having a different polarity from that of the first electric charge, a charge-transport material enabling at least the first electric charge to be transported to isolate the first electric charge and the second electric charge, and a trapping material retaining the first electric charge. The optical characteristics of the recording layer is changed in accordance with changes in spatial distribution of the first and second electric charges, and the trapping material is provided with a conjugated system and with at least one nitrogen-containing heterocyclic group, and bonded through an unsaturated carbon atom of the heterocyclic group to the conjugated system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2001-119979, filed Apr. 18,2001; and No. 2002-087072, filed Mar. 26, 2002, the entire contents ofboth of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical recording medium, in particular, anoptical recording medium having a recording layer where a space chargefield is designed to be formed by the irradiation of beam, and also toan optical recording apparatus which is designed to record informationthrough such an optical recording medium.

2. Description of the Related Art

As a recording medium which is capable of recording data which requiresa large memory capacity such as an image of high density, an opticalrecording medium is known to be useful. Conventionally, as an opticalrecording medium, a photomagnetic recording medium and an optical phasechange recording medium have been developed. However, there is stillincreasing demands for an optical recording medium having a capacity forrecording a more increased density of information.

There has been proposed a holographic memory as an optical recordingmedium for realizing the recording of such an increased density ofinformation. In this holographic memory, a page data where an opticalintensity, polarization, or the phase thereof is two-dimensionallymodulated is interfered with a reference light beam so as to enableinformation to be stored as a hologram in a recording layer. Thisholographic memory is designed such that the thickness of the recordinglayer is made large so as to enable a large quantity of holograms to berecorded in the same overlapped region by slightly changing the incidentangle of the reference light beam or by slightly displacing therecording position.

As for the materials useful for the recording medium of such aholographic memory, the employment of inorganic materials have beenstudied in the past. In recent years however, a photorefractive mediumemploying an organic polymer compound is now being extensively developedbecause of the reasons that it no longer necessitates to manufacture acrystalline material or that it is possible to obviate the difficultiesin the control of characteristics of the crystal (For example, U.S. Pat.No. 5,064,264).

This photorefractive medium is provided with a recording layercontaining a charge-generating material, a charge-transport material, atrapping material and a nonlinear optical material. As a signal beam anda reference beam, both beams interfering with each other, aresimultaneously irradiated onto this recording layer, information isenabled to be recorded therein in the form of interference fringes ofboth beams. When the same reference beam that has been employed in therecording is irradiated onto the recording layer, a reconstructed beamhaving the same spatial characteristics as those of the signal beam isenabled to be read.

Specifically, in the portion of the recording layer that has beenirradiated by the beams, an electric charge is generated from thecharge-generating material and then isolated by the charge-transportmaterial. By enabling this isolated electric charge to be retained bythe trapping material, a space charge field is formed inside therecording layer. Due to this space charge field, the refractive index ofthe nonlinear optical material is caused to change. Therefore, when apair of beams interfering with each other are irradiated onto therecording layer, an intensity pattern of the beams thus irradiated isenabled to be recorded, as a change in refractive index, in therecording layer.

However, when a difference in molecular orbital energy is increasedbetween the trapping material and the charge-transport material so as toprevent the thermal elimination of recorded data due to the thermalde-trapping from the trapping material, it becomes increasinglydifficult to permit the hopping of electric charge from thecharge-transport material to the trapping material. The reason for thisis explained by the charge transport theory based on the small polaronhopping (D. Emin, Adv. Phys. vol. 24, 305–347 (1975)). Therefore, itwould take a long time for enabling an electric charge to be trapped bythe trapping material, thus necessitating a long time for recordinginformation. On the contrary, when an electric charge is enabled to beeasily hopped from the charge-transport material to the trappingmaterial, a difference in energy between the trapping material and thecharge-transport material is caused to minimize, thereby enabling thethermal elimination of recorded data. Namely, the life of record wouldbe shortened.

On the other hand, as an alternative optical recording medium forrealizing the recording of high density of information, there is known amulti-layer optical recording medium (D. A. Parthnopulous and P. M.Rentzepis, Science vol. 245, pp. 843–844 (1989)). This optical recordingmedium is featured, as shown in this publication, in that a recordingbeam is converged at an optional portion within a uniform opticalrecording medium to bring about a change in optical characteristics onlyin the vicinity of the focused point, thus recording information.Therefore, this optical recording medium differs from those where arecording layer and a non-recording layer are alternately laminated.

In this multi-layer optical recording medium also, the employment ofinorganic materials has been studied (Y. Kawata, H. Ishibashi, and S.Kawata, Opt. Lett. vol. 16, pp. 756–758 (1998)). In recent yearshowever, a photorefractive medium employing an organic polymer compoundis now being extensively developed, as mentioned above, because of thereasons that it no longer necessitates to manufacture a crystallinematerial or that it is possible to obviate the difficulties in thecontrol of characteristics of the crystal. According to thisphotorefractive medium, the optical characteristics thereof changes inproportion to the intensity of beam. However, in a recording mediumwhere the optical characteristics thereof changes in proportion to theintensity of beam, the recording region is expanded depthwise.Therefore, the recording regions neighboring to each other are requiredto be sufficiently spaced apart from each other, thereby obstructing theenhancement of densification of information. As a method for narrowingthe distance depthwise between the recording regions, there has beenstudied a method of generating electric charge by two-photon absorption(D. Day, M. Gu, and A. Smallridge, Opt. Lett. vol. 24, pp. 288–290(1999)). However, since this method requires a very strong light sourcefor generating electric charge by two-photon absorption, an ordinarysemiconductor laser can be no longer useful in performing the recordingusing this method.

BRIEF SUMMARY OF THE INVENTION

Therefore, an object of this invention is to provide an opticalrecording medium wherein information is enabled to be recorded as ahologram, and the time required for the recording can be shortened whileensuring a practical recording life.

Another object of this invention is to provide an optical recordingapparatus which is designed to record information through such anoptical recording medium.

Namely, according to one aspect of the present invention, there isprovided an optical recording medium comprising a recording layercontaining a charge-generating material capable of generating anelectron and a hole by light irradiation, a charge-transport materialenabling at least the hole to be transported to isolate the electron andthe hole, and a trapping material retaining the hole, the opticalcharacteristics of the recording layer being changed in accordance withchanges in spatial distribution of the electron and the hole, and thetrapping material being a compound represented by the following generalformula (A):

wherein CB1 is a conjugated system; and R^(a) and R^(b) may be the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group and bonded through an unsaturatedcarbon atom of the heterocyclic group to the conjugated system.

According to another aspect of the present invention, there is providedan optical recording medium comprising a recording layer containing acharge-generating material capable of generating a first electric chargeand a second electric charge by light irradiation, the second electriccharge having a different polarity from that of the first electriccharge, a charge-transport material enabling at least the first electriccharge to be transported to isolate the first electric charge and thesecond electric charge, and a trapping material retaining the firstelectric charge, the optical characteristics of the recording layerbeing changed in accordance with changes in spatial distribution of thefirst and second electric charges, and the trapping material being acompound represented by the following general formula (B):

wherein CB1 is a conjugated system; and R^(c) is a nitrogen-containingheterocyclic group and bonded through an unsaturated carbon atom of theheterocyclic group to the conjugated system.

According to another aspect of the present invention, there is providedan optical recording medium comprising a recording layer containing acharge-generating material capable of generating a first electric chargeand a second electric charge by light irradiation, the second electriccharge having a different polarity from that of the first electriccharge, a charge-transport material enabling at least the first electriccharge to be transported to isolate the first electric charge and thesecond electric charge, and a trapping material retaining the firstelectric charge, the optical characteristics of the recording layerbeing changed in accordance with changes in spatial distribution of thefirst and second electric charges, and the trapping material being acompound represented by the following general formula (C):

wherein CB2 is a conjugated system selected from the groups shown below:

wherein X is nitrogen atom, oxygen atom or sulfur atom; and

R^(c) is a nitrogen-containing heterocyclic group and bonded through anunsaturated carbon atom of the heterocyclic group to the conjugatedsystem; and R^(d) is a nitrogen-containing heterocyclic group and bondedthrough an unsaturated carbon atom of the heterocyclic group to theconjugated system, or may be selected from the group shown below;

wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.

According to another aspect of the present invention, there is providedan optical recording medium comprising a recording layer containing acharge-generating material capable of generating an electron and a holeby light irradiation, a charge-transport material enabling at least thehole to be transported to isolate the electron and the hole, and atrapping material retaining the hole, the optical characteristics of therecording layer being changed in accordance with changes in spatialdistribution of the electron and the hole, and the trapping materialbeing a polymer having, at a side chain thereof, a group represented bythe following general formula (A′):

wherein CB1 is a conjugated system; and R^(a) and R^(b) may be the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group and bonded through an unsaturatedcarbon atom of the heterocyclic group to the conjugated system.

According to another aspect of the present invention, there is providedan optical recording medium comprising a recording layer containing acharge-generating material capable of generating a first electric chargeand a second electric charge by light irradiation, the second electriccharge having a different polarity from that of the first electriccharge, a charge-transport material enabling at least the first electriccharge to be transported to isolate the first electric charge and thesecond electric charge, and a trapping material retaining the firstelectric charge, the optical characteristics of the recording layerbeing changed in accordance with changes in spatial distribution of thefirst and second electric charges, and the trapping material being apolymer having, at a side chain thereof, a group represented by thefollowing general formula (B′):

wherein CB1 is a conjugated system; and R^(c) is a monovalent orbivalent, nitrogen-containing heterocyclic group and bonded through anunsaturated carbon atom of the heterocyclic group to the conjugatedsystem.

According to another aspect of the present invention, there is providedan optical recording apparatus comprising:

a light source emitting a beam;

a beam splitter separating the beam into two beams;

a first optical device which is configured to provide one of theseseparated beams with information to be recorded;

an optical recording medium comprising a recording layer containing acharge-generating material capable of generating an electron and a holeby light irradiation, a charge-transport material enabling at least thehole to be transported to isolate the electron and the hole, and atrapping material retaining the hole, the optical characteristics of therecording layer being changed in accordance with changes in spatialdistribution of the electron and the hole; and

an second optical device which is configured to directing the separatedbeams so as to intersect each other within the recording medium, theintersecting beams making interference fringes within the recordinglayer of the optical recording medium to write information;

wherein the trapping material is a compound represented by the followinggeneral formula (A):

wherein CB1 is a conjugated system; and R^(a) and R^(b) may be the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group and bonded through an unsaturatedcarbon atom of the heterocyclic group to the conjugated system.

According to another aspect of the present invention, there is providedan optical recording apparatus comprising:

a light source emitting a beam;

a beam splitter separating the beam into two beams;

a first optical device which is configured to provide one of theseseparated beams with information to be recorded;

an optical recording medium comprising a recording layer containing acharge-generating material capable of generating a first electric chargeand a second electric charge by light irradiation, the second electriccharge having a different polarity from that of the first electriccharge, a charge-transport material enabling at least the first electriccharge to be transported to isolate the first electric charge and thesecond electric charge, and a trapping material retaining the firstelectric charge, the optical characteristics of the recording layerbeing changed in accordance with changes in spatial distribution of thefirst and second electric charges; and

an second optical device which is configured to directing the separatedbeams so as to intersect each other within the recording medium, theintersecting beams making interference fringes within the recordinglayer of the optical recording medium to write information;

wherein the trapping material is a compound represented by the followinggeneral formula (B):

wherein CB1 is a conjugated system; and R^(c) is a nitrogen-containingheterocyclic group and bonded through an unsaturated carbon atom of theheterocyclic group to the conjugated system.

According to another aspect of the present invention, there is providedan optical recording apparatus comprising:

a light source emitting a beam;

a beam splitter separating the beam into two beams;

a first optical device which is configured to provide one of theseseparated beams with information to be recorded;

an optical recording medium comprising a recording layer containing acharge-generating material capable of generating a first electric chargeand a second electric charge by light irradiation, the second electriccharge having a different polarity from that of the first electriccharge, a charge-transport material enabling at least the first electriccharge to be transported to isolate the first electric charge and thesecond electric charge, and a trapping material retaining the firstelectric charge, the optical characteristics of the recording layerbeing changed in accordance with changes in spatial distribution of thefirst and second electric charges; and

a second optical device which is configured to directing the separatedbeams so as to intersect each other within the recording medium, theintersecting beams making interference fringes within the recordinglayer of the optical recording medium to write information;

wherein the trapping material is a compound represented by the followinggeneral formula (C):

wherein CB2 is a conjugated system selected from the groups shown below:

-   -   wherein X is nitrogen atom, oxygen atom or sulfur atom; and

R^(c) is a nitrogen-containing heterocyclic group and bonded through anunsaturated carbon atom of the heterocyclic group to the conjugatedsystem; and R^(d) is a nitrogen-containing heterocyclic group and bondedthrough an unsaturated carbon atom of the heterocyclic group to theconjugated system, or may be selected from the group shown below;

wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view schematically illustrating an opticalrecording medium;

FIGS. 2A to 2D are diagrams respectively illustrating the principle ofrecording a hologram;

FIG. 3 is a graph for illustrating the changes of energy according tothe molecular geometry of trapping materials;

FIGS. 4A to 4C are graphs respectively illustrating the fluorescentspectrum of a compound having carbazolyl group;

FIG. 5 shows a diagram illustrating the construction of an apparatus forrecording hologram in an optical recording medium according to oneexample of the present invention;

FIG. 6 is a diagram illustrating the construction of an apparatus forreading hologram that has been recorded in an optical recording mediumaccording to one example of the present invention;

FIG. 7 is a diagram illustrating the construction of an apparatus forperforming the multi-layered recording of information in an opticalrecording medium according to one example of the present invention;

FIG. 8 is a graph illustrating the reading data where information hasbeen recorded over multiple layers in an optical recording mediumaccording to one example of the present invention;

FIG. 9 is a graph illustrating the absorption spectrum of a trappingmaterial;

FIG. 10 is a diagram illustrating the method of measuringcharge-retaining capability;

FIG. 11 is a graph illustrating the changes of electric potential at thesurface of a sample;

FIG. 12 is a graph illustrating the changes of electric potential at thesurface of a sample; and

FIG. 13 shows a diagram illustrating the construction of an apparatusfor recording hologram in an optical recording medium according toanother example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As a result of intensive studies on the recording layer of hologramrecording medium, it has been found by the present inventors that thegeometry of molecule having a group capable of capturing electric chargethat has been transported has a great influence on the velocity ofcharge-transport.

First of all, the recording and reading of information will be explainedwith reference to FIG. 1 wherein an optical recording medium accordingto one embodiment of the present invention is illustrated. At first, alaser beam is irradiated onto an optical recording medium having arecording layer 2 to record information therein. At this moment, atransparent electrodes 1 and 3 may be formed in advance on the oppositesurfaces of the recording layer 2 to make it possible to perform therecording while applying a voltage between these electrodes 1 and 3. Thelaser beam is separated into a signal beam 4 accompanying therewithinformation which has been shaped into a spatial intensity distribution,polarization distribution, or a phase distribution, and a reference beam5, this couple of beams being permitted to interfere with each other inthe recording layer. In this manner, the information is recorded, asinterference fringes, in the recording layer 2. When the reading beam 5is irradiated onto the recording layer 2 under the same condition asthat of the reference beam 5, a reconstructed beam 6 having the samespatial characteristics as those of the signal beam can bereconstructed. Therefore, by detecting the intensity distribution orphase distribution of the reconstructed beam 6, the information can beread. Even in the case where a voltage is applied to the opticalrecording medium at the time recording, the voltage is not necessarilyrequired to be applied to the optical recording medium on the occasionof reading the information.

The photorefractive material is a material wherein electric charge isenabled to be generated through the irradiation of beam and to bespatially isolated, thus generating a distribution of electric chargeand the space charge field at this irradiated region, through which therefractive index of the material is caused to change. Next, theprinciple of recording hologram will be explained with reference toFIGS. 2A to 2D. When a beam having an intensity distribution as shown inFIG. 2A is irradiated onto an optical recording medium, a distributionof electric charge is generated in a recording layer as shown in FIG.2B. In ordinary organic photorefractive materials, only the hole ispermitted to be transported, and the distribution of hole becomesspatially uniform as shown in the dotted line in FIG. 2B, provided thatthe following conditions are met. Specifically, the condition is thatthe mean free path in the direction of wavenumber vector of thisinterference fringes is not less than about a half of Λ shown in FIG.2A. As a result, the distribution of total electric charge would becomea state as shown in FIG. 2C. Since an external electric field isgenerally applied to the photorefractive medium, the distribution ofelectric field would become a state as shown in FIG. 2D. Further, in acase where a nonlinear optical material is incorporated in thephotorefractive medium, the refractive index of the medium is caused tochange in accordance with the intensity of electric field shown in FIG.2D. As a result, the changes of refractive index corresponding to thedistribution of the intensity of beam irradiated would be caused to begenerated. In this manner, the spatial intensity distribution,polarization distribution, and/or the phase distribution of the signalbeam 4 are enabled to be recorded as a form of hologram in the recordinglayer.

The principle of the re-distribution of the electric charge that hasbeen generated from the irradiation of beam is consisted of twoprocesses, i.e. the transport of electric charge and the retention ofelectric charge. The charge-transport material is designed to transportthe optically generated electric charge by hopping conduction which canbe effected through the drift and diffusion of electric charge to beenhanced by an external electric field. Further, the trapping materialis designed to capture the electric charge that has been transported bythe charge-transport material. The probability that the electric chargeis hopped from the trapping material to the charge-transport materialshould be smaller than the probability that the electric charge ishopped from the charge-transport material to the trapping material. Inorder to realize this, a difference in molecular orbital energy betweenthe charge-transport material and the trapping material has been takenadvantage of in the past in general. In this case, the molecular orbitalto be compared with each other differs depending on the polarity ofelectric charge to be transported by the hopping. Namely, when electronis to be transported, it is LUMO (Lowest Unocculied Molecular Orbital),whereas when hole is to be transported, it is HOMO (Highest OcculiedMolecular Orbital). Therefore, when electron is to be transported, theLUMO of the charge-transport material should have a larger energy thanthe LUMO of the trapping material, whereas when hole is to betransported, the HOMO of the charge-transport material should have asmaller energy than the HOMO of the trapping material.

The probability that the electric charge hops once from a givencharge-transport material to another charge-transport material differsgreatly depending on the manner in which an electron clouds of moleculesoverlaps with each other as well as on the energy required for forming apolaron. This overlap integral is relatively large between the groupswhere the electron-donating/accepting property thereof is of the samestructure with each other, but is relatively small between the groupswhere the electron-donating/accepting property thereof is quitedifferent in energy and structure from one another. Therefore, in orderto enable electric charge to be rapidly transported from thecharge-transport material to the trapping material, it is preferable toprovide both of the charge-transport material and the trapping materialwith a group having the same kind of electron-donating/acceptingproperty.

As for the trapping material, it is possible to employ a compoundwherein at least one nitrogen-containing heterocyclic group is bonded toa conjugated system. In this case, the nitrogen-containing heterocyclicgroup is required to be bonded to the conjugated system through anunsaturated carbon atom of the heterocyclic group. One example of thecompounds meeting the aforementioned conditions can be represented bythe following general formula (4).

(wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom. Hydrogen atom mentionedanywhere in this specification includes heavy hydrogen.).

The molecular geometry of the compound represented by the generalformula (4) was calculated with respect to the neutral state thereof aswell as with respect to the ionized state thereof, and the bond lengthof each of these states was compared with each other. As a result, itwas found that the ionized state of the compound could be nearlyrepresented by the following general formula (4a).

(wherein R¹ and R² are the same as those of the aforementioned generalformula (4)).

Generally, the relationships between a variable such as the bond lengthor the bond angle and the energy of electric charge can be representedas shown in FIG. 3 wherein the abscissa represents the variable and theordinate represents the energy of electric charge. In FIG. 3, the curve“a” denotes the energy of the neutral state, and the curve “b” denotesthe energy of the ionized state. When the molecular geometry isprominently altered due to the ionization of molecule, the energy of themolecule will be lost in the form of lattice vibration due to thealteration of molecular geometry. Further, there is a great differencein molecular geometry between the neutral state and the ionized state,so that when the conjugated state is enhanced throughout the molecule,the dependency of the energy of electric charge on the moleculargeometry would be further increased in the ionized state as comparedwith that in the neutral state as shown in FIG. 3. Namely, since thetrapping material of ionized state is enabled to return to the neutralstate thereof by releasing electric charge, much more energy is requiredin that case. Because of this, the molecular geometry is caused to alterwhen the molecule is ionized, so that it is possible, by using atrapping material which is capable of enhancing the conjugated statethroughout the molecule, to stably retain the electric charge. Namely,the trapping material exhibiting such features has a prolonged chargeretention life. In order to bring about such a structural change, thetrapping material according one embodiment of the present invention isrequired to have a conjugated system. The conjugated system herein meansa sequence of continuous conjugated bond without being cut off by anon-conjugated bond. Due to the easiness to generate the structuralchange, the trapping material according one embodiment of the presentinvention is provided with a nitrogen-containing heterocyclic group andis required to be bonded to the conjugated system through an unsaturatedcarbon atom of the heterocyclic structure.

By the term “conjugated system”, it means a skeleton (atomic group)having two or more multiple bonds such as —C═C—, a triple bond of carbonatoms, benzene ring of para-bond, —C═N—, pyrrole, oxazole, thiazole,imidazole, 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1H-1,2,4-triazole, or acombination of these groups. More specifically, the bonds shown by thefollowing chemical formulas can be employed.

(wherein X is nitrogen atom, oxygen atom or sulfur atom.).

Especially, it is preferable that the conjugated system includeselectrical active nitrogen atom having an un-paired electron.Alternatively, the employment of phenyl group of para-linkage is alsopreferable, because of the reason that, through the change of statebetween the neutral state and the ionized state thereof, the moleculargeometry thereof can be prominently altered through a change in relativeangle between the terminal groups.

By the way, any of the hydrogen atom in the conjugated system can besubstituted by alkyl group, alkoxy group, phenyl group, naphthyl group,tolyl group, benzyl group, benzothiazole group, benzoxazolyl group,benzopyrrol group, benzoimidazolyl group, naphthothiazolyl group,naphthoxazolyl group, naphthopyrrol group, naphthoimidazolyl group orhydroxyl group. However, in order not to obstruct the geometrical changethroughout the molecule, the substituent group to be incorporated shouldpreferably be as small as possible. For example, it is preferable toemploy alkyl group having not more than two carbon atoms such as methyland ethyl, or hydroxyl group.

Among the aforementioned conjugated system, those containing a cyclicgroup will be restricted with regard to the site to which the terminalgroup containing at least one nitrogen-containing heterocyclic groupwould be bonded. More specifically, the site of conjugated systemcontaining a cyclic group, to which the terminal group can be bonded, isrequired to be such that makes it possible to realize a structuralchange as shown in the aforementioned general formula (4) and generalformula (4′).

This structural change will be explained more in detail with referenceto the following general formulas (6a), (6b), (6c) and (6d)

Since the compound represented by the aforementioned general formula(6a) is a molecule wherein a couple of nitrogen-containing heterocyclicgroups are bonded to each other through a conjugated bond, a structuralchange throughout the molecule such as shown in the aforementionedgeneral formula (6b) can be permitted to take place. The aforementionedgeneral formulas (6c) and (6d) show structures wherein the site to whichthe nitrogen-containing heterocyclic group is bonded is altered. In thecase of the molecule represented by the general formula (6c) or thegeneral formula (6d), since the nitrogen-containing heterocyclic groupis not bonded by a conjugated system, it is impossible to bring about ageometrical change throughout the molecule.

Because of this reason, the trapping material according to thisembodiment of the present invention is required to be provided with aconjugated system.

Further, as for the nitrogen-containing heterocyclic group, it ispossible to employ the following groups.

The nitrogen-containing heterocyclic groups which can be represented bythe aforementioned general formula are generally provided with electrondonativity. Among these nitrogen-containing heterocyclic groups, thegroup represented by the general formula (1c) is especially preferable.In this general formula (1c), the nitrogen atom exhibiting electrondonativity is bonded to the para position of the conjugated systemthrough an unsaturated carbon atom of the phenyl group. Therefore, thenitrogen atom is enabled to easily interact with the conjugated system.Moreover, since saturated carbon atom is included in the heterocyclicstructure thereof, the nitrogen-containing heterocyclic group isrelatively large in free volume and is liable to change the geometrythereof. Namely, in the case of the group represented by the generalformula (1c), since the carbon atoms which constitutes a closed ringstructure of single bond are located away from the plane where thephenyl group is existing, the group is enabled to have a sufficient freevolume. Therefore, when the group represented by the general formula(1c) is permitted to have holes, the molecular geometry of the group canbe easily changed through the conjugated system.

The trapping material according to one embodiment of the presentinvention can be represented by any one of the following generalformulas (A), (B) and (C):

(wherein CB1 is a conjugated system; and R^(a) and R^(b) may the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group and bonded through an unsaturatedcarbon atom of said heterocyclic group to said conjugated system, andwherein hydrogen atom bonded to said heterocyclic group may besubstituted by an another group.);

(wherein CB1 is a conjugated system; and R^(c) is a nitrogen-containingheterocyclic group and bonded through an unsaturated carbon atom of saidheterocyclic group to said conjugated system, and wherein hydrogen atombonded to said nitrogen-containing heterocyclic group may be substitutedby an another group.); and

(wherein CB2 is a conjugated system selected from the groups shownbelow:

(wherein X is nitrogen atom, oxygen atom or sulfur atom.);

R^(c) is a nitrogen-containing heterocyclic group and bonded through anunsaturated carbon atom of said heterocyclic group to said conjugatedsystem; and R^(d) is a nitrogen-containing heterocyclic group and bondedthrough an unsaturated carbon atom of said heterocyclic group to saidconjugated system, or may be selected from the group shown below;

(wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.); and

hydrogen atom bonded to said heterocyclic group included in R^(c) andR^(d) may be substituted by an another group.).

Next, the compounds that can be represented respectively by theaforementioned general formulas (A), (B) and (C) will be explained.

In the compounds represented by the aforementioned general formula (A),a couple of electron-donating groups are bonded to each other through aconjugated system. However, at least one of these electron-donatinggroups is required to be the nitrogen-containing heterocyclic groupexplained above.

Although there is not any particular limitation with respect to theconjugated system which is represented by CB1 in the aforementionedgeneral formula (A), it is more preferable that the conjugated system isprovided with electron donativity. Specific preferable examples of sucha CB1 are as follows.

On the other hand, the other electron-donating group that can beintroduced as R^(a) and R^(b) into the aforementioned general formula(A) may be at least one kind of material selected from the groupconsisting of allyl alkane; nitrogen-containing cyclic compound;oxygen-containing compound; sulfur-containing compound; and thefollowing groups:

(wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.).

Specific examples of the nitrogen-containing cyclic compound are thegroups shown below, indole, carbazole, oxazole, isooxazole, thiazole,imidazole, pyrazole, oxadiazole, pyrazoline, thiazole and triazole. Asfor the oxygen-containing compound, it is possible to employ, forexample, oxazole and derivatives thereof, oxadiazole and derivativesthereof, etc. As for the sulfur-containing compound, it is possible toemploy, for example, thiazole and derivatives thereof, thiadiazole andderivatives thereof.

Among these groups, the hydrogen atom which is bonded to a site whichgives a negligible influence to the charge transport may be substitutedby any other group such as alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group, halogen atom, and hydroxyl group. However, inorder not to obstruct the geometrical change throughout the molecule,the substituent group to be incorporated should preferably be as smallas possible. For example, it is preferable to employ alkyl group havingnot more than two carbon atoms such as methyl and ethyl, or hydroxylgroup. If any of these substituent groups is to be incorporated togetherwith other kinds selected from these groups, they are not necessarily ofthe same kinds.

By the way, even if a couple of electron accepting groups are employedin place of the electron donating groups and bonded to each otherthrough the conjugated system, it would be useful as a trappingmaterial. In this case however, the conjugated system should preferablybe electron-acceptive. One example is represented by the followingformula.

In the compounds represented by the aforementioned general formula (B),a couple of the aforementioned nitrogen-containing heterocyclic groupsare bonded to each other through a conjugated system.

Although there is not any particular limitation with respect to theconjugated system to be introduced as the CB1 in the aforementionedgeneral formula (B), the same kinds of conjugated systems as explainedwith reference to the aforementioned general formula (A) can bepreferably employed. On the other hand, as for the nitrogen-containingheterocyclic group to be incorporated as R^(c) in the aforementionedgeneral formula (B), the following groups can be employed.

Among these groups, since the structural change is easily occurred, thegroup represented by the general formula (1c) is more preferable.

In the case where a couple of the nitrogen-containing heterocyclicgroups that have been bonded to each other through the conjugated systemCB1 are the same with each other, the compound represented by theaforementioned general formula (B) would become symmetrical as a whole.Namely, the compound to be obtained in this manner may be linearlysymmetrical in structure.

In the compounds represented by the aforementioned general formula (C),a nitrogen-containing heterocyclic group R^(c) and a specific groupR^(d) are bonded to each other through a conjugated system. However, theconjugated system to be incorporated as the CB2 in the general formula(C) should be selected from the following groups.

(wherein X is nitrogen atom, oxygen atom or sulfur atom.).

On the other hand, the group that can be introduced as the R^(d) intothe aforementioned general formula (C) may be a nitrogen-containingcyclic compound or a group represented by the following general formula(1a) or (1b):

(wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.).

In addition to the compounds represented by the aforementioned generalformulas (A) to (C), a compound consisting of a molecule having ageometry wherein two or more electron-donative groups are spatiallyoverlapped may be employed as the trapping material.

By the expression of “a geometry wherein two or more electron-donativegroups are spatially overlapped”, it means a geometry wherein two ormore electron-donative groups are respectively formed of a planarstructure, so that when a trapping material is projected in a directionperpendicular to the plane of one of the electron-donative groups, theseelectron-donative groups are observed in a overlapped state, and hence ageometry wherein two or more electron-donative groups are directlyintersected with each other would be excluded from the aforementioneddefinition.

In the case where the electron-donative groups are formed of a planarstructure, even if a couple of the electron-donative groups are notbonded with each other through a conjugated system, the charge retentionproperty of the molecule through the geometrical change thereof can beimproved. This phenomenon can be explained as follows. Namely, if it isassumed that the symmetry axes of the electron-donative groups areslightly offset under a neutral state. In this molecule, when electriccharge is injected into one of these two electron-donative groups, thestability of the molecule under the neutral state is collapsed, so thata structural change is possibly caused to occur so as to align thesesymmetry axes of these two groups with each other. When these symmetryaxes of these two groups are aligned with each other, the interactionbetween these two groups is sharply increased, so that the energy of themolecule is caused to change prominently as compared with the case wherethese two groups existed independently. Therefore, when the molecule isconstructed in this manner, the charge retention property of themolecule can be improved.

This phenomenon will be further explained with reference to thecompounds represented by the following chemical formulas (41) and (42).

The compounds represented by the chemical formulas (41) and (42) areprovided respectively with carbazolyl group as an electron-donatinggroup having a planar structure. Whether or not this carbazolyl groupsis spatially overlapped with another one can be determined by afluorescence spectrum for instance.

The fluorescence spectrums of these compounds are shown in FIGS. 4A and4B. FIG. 4C shows the fluorescence spectrum of N-ethylcarbazole. By theway, the measurement of these fluorescence spectra was performed in asolution of cyclohexane at room temperature and using an exciting lightsource 310 nm in wavelength.

The molecules represented by these chemical formulas (41) and (42) arethe same with each other. However, in the compound represented by thechemical formula (41), carbazolyl groups are spatially overlapped witheach other. Due to this geometrical difference, these two compoundsexhibit quite a different fluorescence spectrum from each other as shownin FIGS. 4A and 4B.

Specifically, in the case of the compound represented by the chemicalformula (42) (FIG. 4B), the compound exhibit the same fluorescencespectrum as that of N-ethylcarbazole (FIG. 4C) excepting that the widthof the spectrum thereof is slightly enlarged and that the wavelengththereof is slightly shifted toward the longer wavelength side.Therefore, it is assumed that in the case of the molecule shown in thechemical formula (42), a couple of carbazolyl groups are not spatiallyoverlapped but are permitted to function independently. On the otherhand, in the case of the molecule represented by the chemical formula(41) (FIG. 4A), the center thereof is more prominently shifted towardthe longer wavelength side than that of the compound shown in FIG. 4C,and a spectrum having a greatly expanded width can be observed. Thisindicates the fact that since a couple of carbazolyl groups arespatially overlapped with each other, an interaction is caused togenerate between these carbazolyl groups.

As explained above, when, as a result of the observation of thefluorescence spectrum of the molecule, the spectrum obtained isrecognized as being different in structure from the spectrum to beobtained where a specific group is existed only one, it can be said thatthere is an interaction between a couple of the specific groups in themolecule.

Examples of such an electron-donating group having a planar structureinclude a nitrogen-containing cyclic compound such as indole, carbazole,isooxazole, imidazole, pyrazole, pyrazoline, triazole, purine, acridine,pyridine, and quinoline; an oxygen-containing compound such as oxdazoleand derivatives thereof, and oxadiazole and derivatives thereof; andsulfur-containing derivatives such as thiazole and derivatives thereof,and thiadiazole and derivatives thereof.

The trapping materials described above are capable of changing theirmolecular geometry through the retention of electric charge, thusresulting in the changes of optical characteristics thereof (i.e.characteristics that can be represented by an optical constant such asrefractive index, absorbance, etc.). In the case of the trappingmaterial where a couple of electron-donating groups or a couple ofnitrogen-containing heterocyclic groups are coupled to each otherthrough a conjugated system, the changes of these opticalcharacteristics can be directly utilized in the optical recording. Inthis case, when an electric charge is generated from a charge-generatingmaterial due to the irradiation of beam, the electric charge istransported and retained in the trapping material. When the trappingmaterial retains the electric charge, the molecular geometry and theoptical characteristics of the trapping material are changed. As aresult, the information is recorded in the recording layer asdistribution of trapping materials retaining the electric charge.Therefore, in the case of the optical recording medium of this kind, theconventional non-linear optical materials whose optical characteristicscan be changed by electric field are no longer necessitated.

Even in the case of the optical recording medium where the recording ofinformation is achieved through the changes in optical characteristic ofthe trapping material as described above, it is possible to perform therecording and reading of information in the same manner asconventionally employed. The principle of the phenomenon to generatechanges in optical characteristics of the optical recording medium inaccordance with the distribution of intensity of irradiated beam will beexplained with reference to FIGS. 2A to 2D.

In contrast with the conventional photorefractive medium, when the meanfree path of hole to be transported is sufficiently small relative to Λ,the distribution of hole does not substantially change. Therefore, thedistribution of electric charge as shown in FIG. 2C would not begenerated. Therefore, the distribution of space charge field as shown inFIG. 2D also is not generated irrespective of the existence ornon-existence of external electric field, so that even in the case ofthe medium containing a non-linear optical material, changes in opticalcharacteristics thereof in conformity with the distribution of theintensity of beam would not be generated. In the case of the opticalrecording medium where the recording of information is achieved throughthe changes in optical characteristic of the trapping material, if thetrapping material represented by the aforementioned general formula (4)is dispersed therein, the trapping material retaining electric charge ispermitted to exist in a large quantity in the region where a largequantity of electric charge is caused to generate due to a strongintensity of beam. The trapping material retains the electric chargeonly in the region in which the light intensity is high, and the opticalcharacteristics of the trapping material are changed. Namely, inconformity with the distribution of intensity of beam, the opticalcharacteristics would be caused to change. In this case, if the meanfree path of electric charge in the direction of wavenumber vectorbecomes almost the same as that of Λ, the distribution of the trappingmaterial retaining electric charge would become uniform as shown in FIG.2C, so that any local change in optical characteristics would not begenerated.

As far as the mean free path of the optically produced electric chargeis concerned, the aforementioned optical recording medium differs fromthose containing a non-linear optical material.

Even in the aforementioned optical recording medium, the compoundsdescribed above can be employed as a trapping material. As mentionedabove, the compound represented by the general formula (4) is enabled tochange the molecular geometry thereof as electric charge is retainedtherein, so that the electric charge can be stably trapped. If thetrapping material contains, in the molecule thereof, two or moreelectron-donating groups or nitrogen-containing heterocyclic groups, acouple of electric charges can be retained therein. In that case, a moreenhanced geometrical change would be brought about as compared with themolecule retaining therein only one electric charge. Specifically, thegeometry would become much close to the state shown by the generalformula (4a), so that the conjugated system would be furtherstrengthened throughout the molecule. Therefore, when a couple ofelectric charges is retained as mentioned above, the life of theretained electric charge would be further prolonged and at the sametime, the changes of optical characteristics would become moreprominent, thereby making it possible to realize an ideal state.

When investigations were performed with regard to the absorption peakwavelength in the neutral state represented by the following formula(16) and in the state wherein a couple of electric charges were retainedin molecule, the following results were obtained. Namely, an absorptionpeak in the neutral state was admitted in the vicinity of 290 nm,whereas an absorption peak in the state wherein a couple of electriccharges were retained in molecule was admitted in the vicinity of 713nm. The alteration of absorption in this manner accompanies, accordingto the relation of Kramers-Kronig, the alteration of refractive index.Therefore, it will be clearly understood that when the changes ingeometry are caused to generate in the trapping material due to theretention of electric charge, the optical characteristics such asabsorption coefficient and refractive index are also caused to change.

However, in the case where a trapping material is dispersed in a matrixpolymer of this optical recording medium, the structural change is notalways taken place at every occasions when the trapping materialreceives electric charge. Generally, when a structural change is causedto occur, electric charge is deprived of its energy and, therefore, isstably retained by the trapping material. Occasionally, the electriccharge may be transported to another electron-donating group before thestructural changes are taken place. Otherwise, the electric charge maybe transported over a plurality of trapping materials. Therefore, theseparation of electric charge to be generated from the charge-generatingmaterial may happen to be generated only from the trapping material too.Of course, it is preferable, for the purpose of effectively generatingthe dissociation of electric charge, to incorporate a charge-transportmaterial in separate from the trapping material.

When a couple of coherent beams are irradiated onto the recording layercontaining such a trapping material, changes in optical constant andhaving the same cycle as the interference fringes would be generatedprovided that the mean free path of the interference fringes of electriccharge in the direction of wavenumber vector is sufficiently smallerthan the Λ. Therefore, when one of the beams generating the interferencefringes is shut off, a diffracted beam can be observed.

Further, when a recording beam is converged on this recording layer, alarge quantity of electric charge would be generated in the vicinity ofthe focal point, so that when the mean free path of electric charge isnot larger than the beam diameter at the beam waist, a greater number ofthe trapping material are enabled to retain electric charge in thevicinity of the focal point. Especially, since the electric charge ispermitted to generate all at once in a very narrow region in thevicinity of focal point in this recording method, the probability ofenabling a single trapping material to retain a couple of electriccharges will be increased. This probability of enabling a singletrapping material to retain a couple of electric charges would beproportional to a square of the density of electric charge, if it isconsidered in a most simple way. On the other hand, since the number ofelectric charge to be generated by the irradiation of beam isproportional to the intensity of beam, the probability of enabling asingle trapping material to retain a couple of electric charges wouldbecome proportional to a square of the intensity of beam. For thisreason, it become possible to bring about changes of the opticalconstant only in a narrower region.

The mixing ratio of the trapping material in the optical recordingmedium may be determined in such a way as to enable an average distancebetween trapping property-provided units to fall within a desired range.The density of the optical recording medium can be determined from theweight and volume of the medium. The amount of the trapping material inthe optical recording medium can be determined from the molecular weightand mixing ratio of the trapping material. On the basis ofabove-mentioned factors, an average distance between trappingproperty-provided units can be determined.

In the case where the recording layer contains a non-linear opticalmaterial, the mean transport path of electric charge is required to benot less than about Λ/2. Therefore, an average distance between trappingproperty-provided units (hereinafter referred to as average inter-unitdistance) should preferably be within the range of 1.0 nm to 4.0 nm.Although it depends on the kinds of trapping material, if the averageinter-unit distance is 1.0 nm, the content of the trapping materialwould become about 40% by weight based on the entire weight of therecording layer, and if the average inter-unit distance is 4.0 nm, thecontent of the trapping material would become about 1% by weight. If thecontent of the trapping material is too small, it would take a long timefor enabling optically generated electric charge to be captured by thetrapping material, thus requiring a long time for the recording. On theother hand, if the content of the trapping material is too large, theoptically generated electric charge would be trapped before the electriccharge is sufficiently isolated, so that it would become difficult toestablish a sufficient magnitude of space charge field.

On the other hand, if it is desired to employ the recording layer as anoptical recording medium where the mean transport path of electriccharge is sufficiently smaller than the Λ, an average inter-unitdistance should preferably be within the range of 0.9 nm to 1.3 nm.Although it depends on the kinds of trapping material, if the averageinter-unit distance is 0.95 nm, the content of the trapping materialwould become about 60% by weight based on the entire weight of therecording layer, and if the average inter-unit distance is 1.3 nm, thecontent of the trapping material would become about 20% by weight If thecontent of the trapping material is too small, the mean free path ofelectric charge would be enlarged, so that it would be difficult togenerate a sufficient degree of changes in optical characteristics inconformity with the distribution of the intensity of beam. On the otherhand, if the content of the trapping material is too large, theaggregation and crystallization among the trapping materials would becaused to generate, so that it may become impossible to form a recordinglayer containing uniformly dispersed molecules. The optically generatedelectric charge is transported by hopping from a charge-generatingmaterial to a charge-transport material, and from one of thecharge-transport materials to another one, thus enabling it to beultimately captured by the trapping material. If the content of thecharge-transport material is too large, the mean free path of electriccharge would be enlarged, so that it would become difficult to generatea sufficient degree of changes in optical characteristics in conformitywith the distribution of the intensity of beam.

By the way, when the trapping material is incorporated into therecording layer so as to enable an average inter-unit distance to fallwithin the range of 1.0 nm to 1.3 nm, the mean transport path ofelectric charge can be adjusted by adjusting the mixing ratio of thecharge-transport material relative to the content of the trappingmaterial.

Followings are explanation on other kinds of materials useful in oneembodiment of the present invention.

The charge-generating material is designed to generate a carrier when itis irradiated with beam. Examples of this charge-generating materialinclude an inorganic photoconductor such as selenium and alloys thereof,CdS, CdSe, CsSSe, AsSe, ZnO, ZnS and amorphous silicon; variouscrystalline type (α, β, γ, δ, ε, ζ, η, θ, ι, κ, λ, μ, ν, ξ, ο, π, ρ, σ,τ, υ, φ, χ, ψ, ω, A, B, C, X, Y) metal phthalocyanine dye such astitanyl phthalocyanine, vanadyl phthalocyanine, and various liquidcrystal type non-metal phthalocyanine dye; azo-based dye and pigmentsuch as monoazo dye, bis-azo dye, tris-azo dye and tetrakis-azo dye;perylene-based dye and pigment such as perylic acid anhydride andperylic imide; perynone-based pigment; indigo-based dye and pigment;quinacridone-based pigment; polycyclic quinone-based pigment such asanthraquinone, anthanthrone and dibromoanthrone; cyanine dye; acharge-transporting complex consisting of an electron-acceptingsubstance and an electron-donating substance, such as TTF-TCNQ; aeutectic complex consisting of pyrylium dye or thiapyrylium dye andpolycarbonate resin; azulenium salt; fullerene and derivatives thereofsuch as C₆₀ and C₇₀; terephthalic acid derivatives having carbonyl groupsuch as dimethyl terephthalate and diethyl terephthalate; xanthene-baseddye and pigment; azulenium dye; and squalylium dye.

These charge-generating materials may be employed singly or incombination of two or more kinds thereof. As described above, two kindsof charge-generating materials which are capable of generating electriccharges differing in polarity, and which can be excited respectively bya different wavelength of beam may be dispersed in the recording layer.In this case, one of these charge-generating materials is employed forerasing.

These charge-generating materials can be incorporated into the recordinglayer, preferably, at a ratio of 0.001 to 40% by weight based on theentire weight of the recording layer. If the content of thecharge-generating material is less than 0.001% by weight, the ratio ofelectric charge to be generated per unit volume by the irradiation ofbeam would become too small, thereby making it difficult to generate asufficient magnitude of space charge field. On the other hand, if thecontent of the charge-generating material exceeds over 40% by weight,the absorption of beam by the charge-generating material would becometoo large, thus preventing the beam from penetrating into the recordinglayer, thereby making it difficult to manufacture an optical recordingmedium having a large film thickness.

As for the charge-transport material, any kinds of material can beemployed as long as it is capable of transporting electric charge byhopping conduction. For example, it is possible to employ π-conjugatetype polymer or oligomer such as polyacetylene, polypyrrole,polythiophene and polyaniline; σ-conjugate type polymer or oligomer suchas polysilane and polygermane; a polycyclic aromatic compound such asanthracene, pyrene, phenanthrene and coronene; a nitrogen-containingcyclic compound or a compound having such a cyclic structure at the mainchain or side chain thereof such as indole, carbazole, oxazole,isooxazole, thiazole, imidazole, pyrazole, oxadiazole, pyrazoline,thiadiazole and triazole; a hydrazone compound; aniline and thederivatives thereof; triphenyl amine; triphenyl methane; butadiene;stilbene; TCNQ; anthraquinone; and diphenoquinone.

Specific examples of these compounds are compounds represented by thefollowing chemical formulas (5a) to (5c), a low molecular compound suchas triphenyl amine, chloroanyl, bromoanyl, tetracyanoethylene,tetracyanoquinodimethene, 2,4,7-trinitro-9-fluorenone,2,4,5,7-tetranitro-9-tetrafluorenone,2,4,7-trinitro-9-dicyanomethylenefluorenone,2,4,5,7-tetranitro-xanthone, 2,4,9-trinitrothioxanthone,N,N-bis(3,5-dimethylphenyl)-3,4,9,10-perylenetetracarboxy imide,diphenone, and stilbenzoquinone, and a polymer having any of thesecompounds at the main chain or side chain thereof.

As for the charge-transport material for transporting hole inparticular, it is preferable, in view of excellent charge-transportingproperty, to employ the compounds represented by the aforementionedchemical formulas (5a) to (5c), or a compound which is slightly modifiedfor facilitating the dissolution thereof in a polymer. The employment ofthese compounds is preferable especially when C₇₀ is employed as acharge-generating material because of its high charge-generatingefficiency.

A polymer represented by the following general formula (8) can be alsoemployed as a charge-transport material.

The polymer represented by the general formula (8) is provided, at themain chain thereof, with a group having a charge-transporting property,so that the polymer is enabled to exhibit an excellentcharge-transporting characteristics. Therefore, when this polymer isemployed in combination with a trapping material according to oneembodiment of the present invention, it is possible to shorten therecording time.

These charge-transport materials can be incorporated into the recordinglayer, preferably, at a ratio of 0.01 to 70% by weight based on theentire weight of the recording layer. The optically generated electriccharge is preferably transported by hopping from a charge-generatingmaterial to a charge-transport material, and from one of thecharge-transport materials to another one, thus enabling it to beultimately captured by the trapping material, thereby generating a spacecharge field. Therefore, if the content of the charge-transport materialis less than 0.01% by weight, the electric charge would be deactivatedin the charge-generating material without being introduced into thecharge-transport material, thereby making it difficult to generate asufficient magnitude of space charge field. On the other hand, if thecontent of the charge-transport material exceeds over 70% by weight, itwould give rise to the aggregation and crystallization among thecharge-transport material, thereby making it difficult to form a devicewherein different kinds of molecules are sufficiently dispersed therein.

Further, when a charge-transport material which is capable of exhibitingan absorption characteristic at the wavelength of irradiated beam isemployed, electric charge is permitted to be generated by theirradiation of beam, thereby enabling the charge-transport material tofunction also as a charge-generating material in this case.

By the way, when it is desired to generate two kinds of electric chargesdiffering in polarity in the optical recording medium, suitable kinds ofcharge-transport materials for transporting these two kinds of electriccharges can be employed. In this case, one of the charge-transportmaterials functions as a charge-transport material for erasing.

As for the non-linear optical material, it is possible to employ, forinstance, 1) a substance which is capable of changing the absorptioncoefficient or refractive index thereof due to Franz-Keldysh effect; 2)a substance which is capable of changing the absorption coefficient,refractive index or luminous efficiency thereof due to exciton effect;3) a substance which is capable of changing the refractive index thereofdue to Pockels effect; and 4) a substance which is capable of changingthe optical characteristics of the excitation state thereof and alsocapable of prolonging the life of the excitation state thereof due to anelectric field.

More specifically, it is possible to employ the following compounds.Namely, it is possible to employ spirobenzofuran type molecule, fulgidemolecule, cyclofen molecular, diaryl ethene type molecule, azobenzenetype molecule, a molecule which is capable of exhibiting photochromismsuch as polryacrylate or polysiloxane each having cyanobiphenyl groupwherein a photochromic molecule is included in a polymer liquid crystal,or polysiloxane containing spirobenzofuran group; and a material whichis capable exhibiting a liquid crystal such as p-azoxy ethyl benzoate,ammonium oleate, and p-azoxy anyl.

Additionally, it is also possible to employ urea and derivativesthereof; thiourea and derivatives thereof; nitrobenzene; carbonylbenzene; π-conjugated benzene derivatives such as benzene sulfonate;pyridine N-oxide; pyridine derivatives such as nitropyridine;π-conjugate type polymer or oligomer such as polyacetylene, polypyrrole,polythiophene and polyaniline; σ-conjugate type polymer or oligomer suchas polysilane and polygermane; a polycyclic aromatic compound such asanthracene, pyrene, phenanthrene and coronene; a nitrogen-containingcyclic compound or a compound having such a cyclic structure at the mainchain or side chain thereof such as indole, carbazole, oxazole,isooxazole, thiazole, imidazole, pyrazole, oxadiazole, pyrazoline,thiadiazole and triazole; a hydrazone compound; triphenyl amine;triphenyl methane; benzene amine; butadiene; stilbene; orphene; imine;piperonal; TCNQ; anthraquinone; diphenoquinone; and fullerene andderivatives thereof such as C₆₀ and C₇₀. Furthermore, it is alsopossible to employ an inorganic photoconductor such as selenium andalloys thereof, CdS, CdSe, AsSe, ZnO, and amorphous silicon; aphthalocyanine dye and pigment such as metal phthalocyanine andnon-metal phthalocyanine dye; azo-based dye such as monoazo dye, bis-azodye, tris-azo dye and tetrakis-azo dye; perylene-based dye and pigment;indigo-based dye and pigment; quinacridone-based dye and pigment;polycyclic quinone-based pigment such as anthraquinone and anthanthrone;cyanine dye; a charge-transporting complex consisting of anelectron-accepting substance and an electron-donating substance, such asTTF-TCNQ; a eutectic complex consisting of pyrylium dye andpolycarbonate resin; and azulenium salt.

These non-linear optical materials may be employed singly or incombination of two or more kinds thereof. These non-linear opticalmaterials can be incorporated into the recording layer, preferably, at aratio of 0.01 to 80% by weight based on the entire weight of therecording layer. If the content of the non-linear optical material isless than 0.01% by weight, it would become difficult to expect asufficient magnitude of change in the optical characteristics thereof.On the other hand, if the content of the non-linear optical materialexceeds over 80% by weight, it would give rise to the aggregation andcrystallization among the non-linear optical materials, thereby makingit difficult to form a device wherein different kinds of molecules aresufficiently dispersed therein.

The charge-generating material, the charge-transport material, thetrapping material, and optionally, the non-linear optical material canbe suitably combined together so as to obtain a desired mean free path.Further, it is also possible to employ a compound whose molecule iscapable of exhibiting two or more functions. In that case, the compoundcan be incorporated without being restricted by the aforementionedmixing ratio thereof (i.e. weight %). Examples of such a compoundinclude the polymers represented by the following chemical formula (P1).

In this polymer represented by the chemical formula (P1), the main chainthereof is constituted by two kinds of units. A first unit is provided,at a side chain thereof, with carbazole group which is good incharge-transporting property, whereas a second unit is provided, at aside chain thereof, with a group wherein a couple of carbazole groupsare coupled with each other through a conjugated system. This-secondunit exhibits a high charge retentivity. Namely, in the polymerrepresented by the chemical formula (P1), the group (trapping material)represented by the aforementioned general formula (B′) is introducedinto a side chain of the compound. The ratio between the first unit andthe second unit (y/x) should desirably be adjusted so as to obtain adesired mean free path. For example, the ratio (y/x) may be confinedwithin the range of 0.001 to 0.5. When the ratio (y/x) is confinedwithin the range of 0.1 to 5, the mean free path can be further reduced.Therefore, it is possible to manufacture an optical recording mediumwithout necessitating the employment of the aforementioned non-linearoptical material.

The group represented by the aforementioned general formula (A′) may beintroduced as a side chain into the main chain of the polymer. Further,the compound represented by the aforementioned general formula (C) maybe introduced, via R^(c) or R^(d), into the main chain of the polymer.

When the trapping material is introduced as a side chain into thepolymer in this manner, the aggregation of the trapping material can beprevented. Therefore, the trapping material can be added at a highconcentration to the polymer. Further, when the trapping material isintroduced as a side chain into the polymer, it become possible toobtain a large magnitude of free volume, thus advantageouslyfacilitating the generation of changes in geometry thereof on theoccasion when electric charge is retained by the trapping material.

By the way, when these components such as the charge-transport materialare not constituted by a polymer, a polymer can be mixed with thesecomponents. The polymer useful in this case should preferably beoptically inactive and be minimal in scattering of molecular weight.However, there is not any particular limitation with respect to thefeatures of the polymer. Examples of the polymer useful in this caseinclude polyethylene resin, nylon resin, polyester resin, polycarbonateresin, polyarylate resin, butyral resin, polystyrene resin,styrene-butadiene copolymer, polyvinyl acetal resin, diallyl phthalateresin, silicone resin, polysulfone resin, acrylic resin, polyvinylacetate, polyolefin oxide resin, alkyd resin, styrene-maleic anhydridecopolymer, phenol resin, vinyl chloride-vinyl acetate copolymer,polyester carbonate, polyvinyl chloride, polyvinyl acetal, polyallylateand paraffin wax. These resins can be employed singly or in combinationof two or more kinds.

In order to lower the glass transition point of the recording layer, aplasticizer, i.e. a molecule of relatively small molecular weight can bedispersed in the recording layer. When the glass transition point of therecording layer is lowered in this manner, the geometry of the trappingmaterial can be more easily changed.

Further, compounds which are generally known as useful as a polymericanti-oxidant or as an ultraviolet absorbent may be employed togetherwith the aforementioned components. Examples of such a compound includehindered phenol, aromatic amine, organosulfur compound, phosphite,chelating agent, benzophenone, benzotriazole, and nickel complex. Themixing ratio of these components should preferably be within the rangeof 0.0001 to 5% by weight.

The recording layer of the optical recording medium according to oneembodiment of the present invention can be formed by a process wherein acomposition comprising the aforementioned components is dissolved in asolvent to obtain a solution, which is then deposited on a substrate. Asfor the solvent useful in this case, it is possible to employ variouskinds of organic solvent. For example, it is possible to employ alcohol,ketone, amide, sulfoxide, ether, ester, halogenated aromatichydrocarbon, and aromatic hydrocarbon.

The recording layer can be formed by using various methods such as aspin-coating method, a dipping method, a roller coating method, a spraycoating method, a wire bar coating method, a blade coating method, and aroller coating method; a casting method; a vacuum deposition method; anda sputtering method. Further, a plasma CVD method using glow dischargemay be employed for forming the recording layer. If a casting method isto be employed, the method may be performed in such a manner that asolution is cast at first, and after the solvent is permitted toevaporate, the residual powder-like material is thermally fused to forma recording layer.

The thickness of the recording layer thus formed may be generally in therange of 0.05 to 10 mm, more preferably 0.5 to 1 mm. By the way, thethickness of the recording layer can be suitably selected by taking intoaccount the characteristics needed for the optical recording layer suchas the recording capacity, transmittance, etc. and also taking intoaccount the composition of the recording layer.

A solution containing a composition comprising the aforementionedcomponents is coated on a suitable substrate to form a recording layer.The substrate useful in this case can be selected from those having asuitable thickness, a suitable hardness and a sufficient strength forthe convenience of handling.

This substrate can be used not only for forming the recording layer butalso as a substrate of the optical recording medium. Namely, the opticalrecording medium according to one embodiment of the present inventioncan be constituted by a substrate and a recording layer formed on thissubstrate. The substrate to be used in this case should preferably betransparent at least to some extent at a wavelength range of beam to beemployed. By the way, the wavelength of beam is, for example, 780 nm,650 nm or 405 nm if a semiconductor laser is to be employed. In the caseof the ordinary resins, they are transparent to a light having awavelength ranging from 400 nm to 600 nm or a visible zone, and many ofthe resins are also transparent to a light having a wavelength up to 800nm or a long wavelength zone. Therefore, polyvinyl chloride,polyvinylidene chloride, polyethylene, polycarbonate, polyester,polyamide, acrylic resin and polyimide can be preferably employed.

These materials for the substrate may be employed as a flat sheet or asa cylindrical sheet if desired. If these materials are employed as acylindrical sheet, the substrate should have a suitable degree offlexibility.

The recording layer disposed on the substrate may be provided, on thesurface thereof, with an electrode layer if desired. As for the materialfor constituting the electrode layer, it should be transparent to somedegree, just like the substrate, in the wavelength range of light to beemployed. Therefore, indium tin oxide or aluminum can be preferablyemployed. The sheet resistance of the electrode should preferably be 50Ωcm⁻² or less.

If desired, the recording layer may be covered with a protective layer.As for the materials for this protective layer, it may be optionallyselected. For example, it is possible to employ a thermosetting resinsuch as acrylic resin, fluororesin, silicone resin and melamine resin; aphotocurable resin, an EB curable resin, an X-ray curable resin and a UVcurable resin.

An additive such as anti-oxidant, an ultraviolet absorbent and ananti-aging agent may be incorporated, at a small amount, in thisprotective layer. Examples of such an additive include hindered phenol,aromatic amine, organosulfur compound, phosphite, chelating agent,benzophenone, benzotriazole, and nickel complex.

When a non-linear optical material is incorporated into the recordinglayer, the recording layer may be subjected in advance to a polingtreatment. In this poling treatment, the recording layer is heated up tonear the glass transition point thereof and an external electric fieldis applied to the recording layer. By doing so, molecules which arerelatively large in permanent dipole moment, in particular, thenon-linear optical material are aligned in the direction of the externalelectric field, thereby enhancing the non-linearity thereof to the beam.

Next, the procedures for using the aforementioned optical recordingmedium as a hologram memory, more specifically, the methods of recordinginformation in the medium and of reading the recorded information willbe explained.

FIG. 5 shows one example of the recording apparatus for recordinginformation in the optical recording medium according to one embodimentof the present invention.

As shown in FIG. 5, an optical recording medium 7 shaped into arectangular parallelepiped is prepared at first. Then, an image displayelement 15 is disposed on one side of the optical recording medium 7,and a reading device 19 is disposed on the opposite side of the opticalrecording medium 7. It is desirable that this reading device 19 isdisposed perpendicular to the axis of beam to be irradiated from animage display element 15 onto the optical recording medium 7. As forthis image display element 15, various kinds of device such as a liquidcrystal, a digital mirror array, a Pockels readout optical modulator, amulti-channel spatial modulator, a Si-PLZT element, a deformed surfacetype element, an AO or EO modulating element and a magneto-opticaleffect element can be employed. As for the reading device 19, any kindof photoelectric converting device can be employed. For example, it ispossible to employ a CCD, a CMOS sensor, a photodiode, a photoreceptorand a photomultiplyer tube.

Although the beam is assumed to pass through the image display element15, this image display element 15 may be constituted by an element whichis designed to reflect the beam.

The recording of information to the optical recording medium can beperformed according to the following procedures. As for the light sourcefor the recording, it is required to be a coherent light which isrepresented by a laser. Therefore, an embodiment where a laser isemployed will be explained herein. The wavelength of the laser can beselected depending on the components of the optical recording medium tobe employed. More specifically, the wavelength of the laser can beselected according to the charge-generating material. In the case wherethe optical recording medium in which the phenomenon changing theoptical characteristics when the trapping material retains charge isused, the wavelength is selected corresponding to the trapping material.As for the laser 10, any kind of conventional laser, such as gas laser,liquid laser, solid laser, or semiconductor laser can be employed. Theoutput emitted from the laser 10 is split into two by using a beamsplitter 12 for instance. Namely, one of them is employed as a referencebeam 5, and the other is employed as a signal beam 4, thus enabling itpass through the image display element 15.

In the recording procedures of information using this apparatus, thesignal beam 4 and the reference beam 5 are irradiated onto the opticalrecording medium 7 so as to enable these beams to intersect with eachother in the recording layer. Specifically, this can be achieved by thefollowing procedures. Namely, the beam emitted from the laser 10 isexpanded to a parallel beam by a beam expander 11, and then, split intotwo by using a beam splitter 12 for instance. The information to berecorded is digitized in advance, and the image pattern corresponding tothis digitized information is input in advance into the image displayelement 15. One of the beam that has been split by the beam splitter 12is irradiated, via the mirror 13, onto the image display element 15 soas to spatially modulate its intensity distribution for instance inaccordance with data for recording, thereby making it the signal beam 4.Further, this signal beam 4 is converged by the lens 16 and irradiatedonto the optical recording medium 7. If the focal distance of the lens16 is defined as f1, the distance between the image display element 15and the lens 16 should preferably be adjusted identical with this f1.Concurrently, the reference beam 5 is irradiated onto the opticalrecording medium 7 so as to enable the reference beam 5 to intersectwith the signal beam 4. By the way, the reference beam 5 is reflectedwith a mirror 14 and then converged in advance by the lens 17.

Due to the interference fringes that has been generated by theoverlapping of the signal beam 4 with the reference beam 5, a spacecharge field is caused to generate. As a result, the modulation ofoptical characteristics is caused to occur in the recording layer, thusforming a diffraction grating. On this occasion, by changing theincident angle of the reference beam and/or the incident angle of thesignal beam, a plurality of interference fringes can be formed in theoverlapping region. Alternatively, by rotating the optical recordingmedium 7 relative to the direction of incident beam, the incident anglesof the reference beam and the signal beam can be varied. Furthermore,when the position to which the laser beam is irradiated is displaced bya magnitude of ½ to 1/1000 of the overlapping region of the signal beamand reference beam, a plurality of interference fringes can be formed inthe overlapping region of two beams.

On the occasion of reading the information that has bee recorded, thesignal beam 4 was shut off, and only the reference beam 5 is irradiatedonto the optical recording medium 7. Namely, the reference beam 5 can beemployed also as reading beam. On this occasion, a reading beam havingthe same spatial intensity distribution as that of the signal beam 4 canbe reconstructed due to the interference fringes that have beenrecorded. Therefore, after being permitted to pass through the lens 18,this reconstructed beam is read out by the reading device 19. From theintensity distribution of the beam read out in this manner, readingrecorded information can be realized. If the focal distance of the lens18 is defined as f2, the distance between the lens 16 and the lens 18should preferably be made equal to f1+f2, and the distance between thelens 18 and the reading device 19 should preferably be made equal to f2.The recording apparatus according to one embodiment of the presentinvention may be provided with a detector detecting a beam to reproducethe information that has been recorded as the interference fringes, suchas the reading device 19. The recording apparatus according to oneembodiment of the present invention may be applied to a readingapparatus only to reproduce the information that has been recorded asthe interference fringes in the above optical recording mediumcomprising the above trapping material, when it is provided with adetector such as the reading device 19.

In this embodiment, the wavelength of the light source employed in thereading is the same as that employed in the recording. However, it isalso possible to employ a light source of different wavelength. Namely,when the thickness of recording layer is not more than 0.5 mm, even if alight source of slightly different wavelength from that employed in therecording is employed in the reading, the reading of the recordedinformation can be realized. In that case, the angle of the reading beammay be slightly altered from that of the recording beam so as to enhancethe intensity of the diffraction beam. Even in this case, the readingdevice 19 should preferably be disposed perpendicular to the axis of thereconstructed beam.

In this embodiment, the reference beam 5 is also converged by the lens17. However, the reference beam may not necessarily be converged.Namely, the lens 17 may be omitted by interposing the beam expander 11at any location between the laser 10 and the image display element 15.

It is also possible to perform phase conjugated reading on the occasionof reading the recorded information. The process thereof will beexplained with reference to FIG. 6. In FIG. 6, the coherent beam of thesame wavelength as that employed in the recording is irradiated in theopposite direction to that employed in the recording.

Namely, the beam emitted from the laser 20 which is designed to emit thesame wavelength as that employed in the recording is permitted to expandin diameter by the beam expander 21 for instance, and then, irradiated,by using the lens 22, onto the optical recording medium 7 from thedirection which is quite opposite from that when the reference beam wasirradiated thereto. As a result, by the diffraction grating that hasbeen recorded in the optical recording medium 7, a virtual image whichis opposite in direction from that forwarded by the signal beam 4 willbe reconstructed. After being passed through the lens 16, the virtualimage is permitted to reflect by the beam splitter 23 so as to be readby the reading device 19. In the same manner as on the occasion ofrecording, the distance between the lens 16 and the reading device 19should preferably be made equal to the focal distance of the lens 16. Itis also possible in this phase conjugated reading to employ a coherentbeam having a slightly different wavelength from that employed in therecording. In that case, the incident angle of the reading beam shouldpreferably be slightly adjusted so as to make the beam axis of thevirtual image completely align with the axis of signal beam 4.

If the reference beam 5 is not converged on the occasion of recording,it is possible to omit the beam splitter 21 and the lens 22 even in thisphase conjugated reading.

The information recorded in the optical recording medium may be erasedif desired. For example, by irradiating a beam having a uniformintensity distribution all over a larger region as compared with therecording region, or by heating the optical recording medium up to atemperature lower than the glass transition temperature thereof, therecorded information can be erased. Alternatively, the recordedinformation can be erased by uniformly irradiating a beam having awavelength against which the trapping material is incapable of absorbingif it is in the neutral state but is capable of absorbing if it is inthe ionized state.

The methods of recording and reading information in the opticalrecording medium according to one embodiment of the present inventionare not confined to the aforementioned examples but can be variouslymodified. For example, the signal beam 4 and the reference beam 5 may beintroduced into the optical recording medium 7 from the different sidethereof, respectively.

If the information is recorded as a digital data, a plurality of pixelsof the image display element 15 may be represented as a single data.

If the information is to be supplied by the intensity distribution ofsignal beam, the intensity of beam at the bright portion and at the darkportion may not be uniform throughout the diameter of beam. Namely, thetransmittance of beam in the image display element may be lowered at thecentral portion thereof and enhanced at the marginal portion. It ispossible in this manner to preliminarily correct the phenomenon that thereconstructed beam is weakened at the marginal region as compared withthe central region. Alternatively, a beam intensity modulating elementhaving a higher absorption coefficient at the central region and a lowerabsorption coefficient at the marginal region may be disposed in frontof the reading device 19 so as to obtain the same effect as mentionedabove.

Next, the method of recording information in a plurality layers of therecording medium and the method of reading information therefrom in themulti-layer optical recording medium according to one embodiment of thepresent invention will be explained with reference to FIG. 7.

The beam emitted from a semiconductor laser 31 is turned into a parallelbeam by a collimator lens 32, and then, converged in the recording layerby using an objective lens 33. By increasing the injection current tothe semiconductor laser 31 for a suitable period of time on the occasionof irradiating laser beam, the information can be recorded as a changein optical constant at only the vicinity of focal point. By locating theposition of focal point at a desired portion within the medium, theinformation can be recorded as the region 34 where the optical constantis altered, which can be distinguished from the region where the opticalconstant is not altered.

The information thus recorded can be read by a process wherein an iris35 is disposed on the axis of beam and the intensity of beam that haspassed through the iris 35 is measured by a beam detector 36. Namely,the reading beam is irradiated so as to enable the axis of reading beamto pass through the region where the optical constant is altered, andthe focal point is scanned in the depth-wise of the recording layer. Asa result, the intensity I of transmitted beam to be observed can beexpressed as a function of the focus position Z in the direction ofdepth (herein, the center of the region where the optical constant wasaltered is defined as Z=0) as shown in FIG. 8. The reason for this canbe attributed to the fact that the refractive index of the medium isaltered in the vicinity around Z=0, thereby generating the lens effect.Even if the region where the optical constant is not altered is scanned,the intensity of transmitted beam would not be changed. Therefore, bydetermining if there is a change in intensity of the transmitted beam,it become possible to determine the region where the optical constant isaltered. Therefore, if the region to be recorded has been determined inadvance, it is possible to read the recorded information by knowing thefluctuation of intensity of transmitted beam in the vicinity of thatpredetermined region.

However, in order to prevent the optical characteristics from beingnewly altered on the occasion of reading information, the injectioncurrent should preferably be controlled so as to confine the intensityof reading beam to about ½ to 1/100 of the recording beam.

Alternatively, as a more simple reading method, the recorded data on therecorded position may be read by detecting the intensity of thetransmitted beam in the vicinity of the recorded position. Namely, therecorded information can be read by detecting the intensity of thetransmitted beam of the position which is displaced in depth-wise by adistance Z₀ (shown in FIG. 8) from the data-recorded position. When thefocal point of reading beam is existed at the position which isdisplaced in depth-wise by the distance Z₀ from the region where theinformation is not recorded, the intensity of the transmitted beam wouldindicate a value I₀, whereas when the focal point of reading beam isexisted at the position which is displaced in depth-wise by the distanceZ₀ from the region where the information is recorded, the intensity ofthe transmitted beam would be I₀+ΔI, so that, by detecting thisintensity of the transmitted beam, the recorded information existing atthe position which is displaced by the distance Z₀ from the focal pointof the reading beam can be read.

In the case where the film thickness is sufficiently large or where theglass plate is disposed on the objective lens side of the recordinglayer, a lens which is long in operating distance such as CF IC LCD PlanCR (tradename, NIKON Co., Ltd.; magnification: 100 times) can bepreferably employed as an objective lens. Further, in the case where thesubstrate is disposed at the top and bottom surfaces of the recordinglayer, the thickness of the substrate to be disposed on the objectivelens side should preferably be 0.5 mm or less. Alternatively, thesubstrate may not be disposed on the objective lens side of therecording layer. In that case, the recording layer may be housed insidea diskette in order to prevent the recording layer from being scratched.

The method of recording information to the optical recording mediumaccording to one embodiment of the present invention as well as themethod of reading the information therefrom are not limited to theaforementioned examples, but may be variously modified. For example,reading information may be performed in the same manner as in the caseof a confocal microscope. Alternatively, a reflector may be provided onthe side of optical recording medium which is opposite to where theobjective lens is disposed, thereby making it possible to readinformation by using a reflected beam.

The optical recording medium according to the present invention is notlimited to the aforementioned embodiments, but may be varied within thespirit of the invention. For example, if the recording layer of theoptical recording medium has a sufficient strength, it may be employedsingly without supplementing it with a supporting plate.

The optical recording medium according to one embodiment of the presentinvention is constructed such that the recording layer thereof containsa conjugated system and at least one nitrogen-containing heterocycliccompound, and that this nitrogen-containing heterocyclic compound iscoupled, through an unsaturated carbon atom, with the conjugated system.Therefore, the optical recording medium of one embodiment of the presentinvention is advantageous in that the response speed thereof is fasterand the recording time is shorter as compared with the conventionaloptical recording medium.

First of all, the features of the trapping material according to oneembodiment of the present invention will be explained in comparison withthe conventional trapping material.

FIG. 9 shows the absorption spectra of the compound represented by thefollowing chemical formula (16) and the compound represented by thefollowing chemical formula (51). In FIG. 9, the curve “c” denotes theabsorption spectrum of the compound represented by the chemical formula(16), and the curve “d” denotes the absorption spectrum of the compoundrepresented by the chemical formula (51).

The compound represented by the chemical formula (16) is a trappingmaterial according to one embodiment of the present invention, whereasthe compound represented by the chemical formula (51) is a trappingmaterial set forth in the publication: “Photoreflective polymers withlow intrinsic trap density”, H. J. Bolink, V. V. Krasnikov, and G.Hadziioannou, Proc. SPIE, Vol. 3144, p.p. 124–133 (1997).

As clearly shown in FIG. 9, the compound represented by the chemicalformula (51) (curve “d”) shows a stronger absorption in the 500–700 nmwavelength range as compared with the compound represented by thechemical formula (16) (curve “c”). Therefore, if a light source of 633nm or 532 nm is to be employed as a light source for the recording beam,it would give rise to the defect that, when the thickness of therecording layer of optical recording medium is enlarged, the intensityof diffracted beam is caused to decrease due to the absorption of thetrapping material. Even in the case of the compound wherein anelectron-donating group having a heterocyclic structure is coupled,through a conjugated system, with an electron-accepting group, itfrequently exhibits a strong absorption at the visible range (forexample, “Effect of the chromophore donor group and ferrocene doping onthe dynamic range, gain, and phase shift in photorefractive polymer”, E.Hendrickx, D. V. Steenwinckel, A. Persoons, C. Samyn, D. Beljonne, andJ-L. Bredas, J. Chem. Phys. vol. 113, p.p. 5439–5447 (2000).

Next, the charge retentivity of the compound represented by theaforementioned chemical formula (16) and of the compound represented bythe following chemical formula (52) was measured by the Xerographicmethod.

The compound represented by the chemical formula (52) is the same instructure as that of the compound represented by the chemical formula(16) excepting that the nitrogen-containing heterocyclic group is notcoupled with the conjugated system.

The method of measuring the charge retentivity will be explained withreference to FIG. 10. First of all, a sample is prepared at a suitablemixing ratio. On the other hand, an ITO film 112 and an Al film 113 aresuccessively evaporated on the surface of a glass plate 111. Then, thesample is deposited on the Al film 113 by using a spinner to form ameasuring film 114. Thereafter, the surface of the measuring film 114 ispositively charged by corona discharge, and then, beam is irradiated tothe surface of the measuring film 114. The electric charge generated bythe irradiation of beam is then transported to an earthed counterelectrode side. If electron is retained by the charge-generating siteafter electric charge has been generated in the vicinity of the surfaceof measuring film 114, the electric potential of the surface of samplewill be attenuated in proportion to the number of hole that has beentransported. More specifically, as indicated by the curve “e” in FIG.11, the electric potential will be attenuated. On the other hand, if thetrapping material for retaining hole is dispersed inside the sample, thetransported distance of hole will be shortened as the hole is retainedby the trapping material. Therefore, the surface potential would not beattenuated down to zero, but is gradually shifted to a constant value asrepresented by the curve “f” in FIG. 11. Therefore, the value of surfacepotential thus shifted would become a hint for knowing the number oftrapped hole. On this occasion, the dark attenuation of the surfacepotential after the finish of irradiation may be considered as beingcaused by the de-trapping of hole that has been once trapped. 25% byweight of the compound shown in the chemical formula (5b), 69.5% byweight of polystyrene, 0.5% by weight of C₇₀, and 5% by weight of atrapping material were dissolved in toluene and tetrahydrofuran toprepare a solution. As for the trapping material, the compoundsrepresented by the aforementioned chemical formulas (16) and (52), andthe compound represented by the following chemical formula (53) wereindividually employed.

Then, by using the solution and by a film-forming method using aspinner, samples each having a thickness of about 5 nm weremanufactured. These samples were electrified to +200V or so, and then,irradiated with a beam having a wavelength of 500 nm for 60 seconds, thedark attenuation of the surface potential on this occasion is shown inFIG. 12.

In FIG. 12, the plots g, h and i show the results tested of the samplescontaining, as a trapping material, the compounds represented by thechemical formulas (16), (52) and (53), respectively.

As shown in FIG. 12, the sample wherein the compound represented by thechemical formula (16) was dispersed therein (plot g) exhibited a lowerattenuation of surface potential as compared with that of the samplewherein the compound represented by the chemical formula (52) wasdispersed therein (plot h). The compound represented by the chemicalformula (16) is featured in that the heterocyclic structure (julolidinegroup) exhibits a stronger donativity as compared with the compoundrepresented by the chemical formula (52). Therefore, this high chargeretentivity of this compound represented by the chemical formula (16)can be easily understood.

Likewise, the compound represented by the chemical formula (16) wascompared, with respect to the charge retentivity, with the compoundrepresented by the chemical formula (53). The compound represented bythe chemical formula (53) is featured in that a group having aheterocyclic group exhibiting a strong donativity is bonded to triphenylamine exhibiting high charge transporting ability via non-conjugatedbond.

As clearly shown in FIG. 12, the sample wherein the compound representedby the chemical formula (16) was dispersed therein (plot g) exhibited alower attenuation of surface potential as compared with that of thesample wherein the compound represented by the chemical formula (53) wasdispersed therein (plot i).

In the case of the compound represented by the chemical formula (53), acouple of electron-donating groups are coupled with each other notthrough a conjugated system. Therefore, even if hole is retained in thiscompound, there is no possibility that the changes in conjugated stateor in molecular geometry can be brought about throughout the molecule.Therefore, the compound represented by the chemical formula (53) islower in charge retentivity as compared with the compound represented bythe chemical formula (16).

By the way, if the conjugated system is too long, it would becomedifficult to expand a conjugated state throughout the molecule even ifelectric charge is retained therein. If such is the case, it wouldbecome difficult to realize a high charge retentivity. Therefore, theconjugated system should preferably be as short as possible.

Next, the present invention will be further explained with reference tothe following examples and comparative examples.

EXAMPLE 1

First of all, an optical recording medium was prepared as follows.

0.5% by weight of fullerene (C₇₀), 44.5% by weight of poly(N-vinylcarbazole) (PVK) as a transport material, 7.5% by weight of N-ethylcarbazole (EtCz) and 7.5% by weight of Bis-carbazolyl propane (BisCzPro)as plasticizing agents, 35% by weight of N-[[4-(dimethylamino)phenyl]-methylene]-2-methyl-4-nitrobenzene amine (DBMNA) as a nonlinearoptical material, and 5% by weight of a trapping material represented bythe following chemical formula (11) were dissolved in a mixture oftetrahydrofuran (THF) and toluene to prepare a co-solvent solution. Asshown in the chemical formula, this compound was provided with a coupleof carbazole groups both exhibiting electron donativity and beingcoupled to each other through a conjugated group.

Then, an ITO (Indium Tin Oxide) film was formed on the surface of aglass plate to prepare a substrate. The aforementioned co-solventsolution was then coated on the surface of this substrate by a castingmethod to form a recording layer. The thickness of this recording layerwas adjusted to 50 μm by using a Teflon (registered trademark) spacer.Further, this recording layer was subjected to a poling treatmentwherein both electrodes thereof were connected with a power source of 3kV at a temperature of 80° C. to prepare an optical recording medium.

By using a recording apparatus constructed as shown in FIG. 13, therecording and reading hologram were performed as follows. In theoperation of this recording apparatus shown in FIG. 13, the beam emittedfrom a He—Ne laser (output: 30 mW) 10 was at first split into two by abeam splitter 12. One of the beams which was reflected by the beamsplitter 12 was passed through a beam expander 11 to expand the diameterof the beam and then, permitted to pass through a liquid crystal filter15 functioning as an image display element. This liquid crystal filter15 was designed such that the transmissivity thereof was modulated inadvance in conformity with the information to be recorded, and that thetransmitted beam was turned into a signal beam 4, which was thenconverged by a lens 16 (focal length: 150 mm). The distance between thelens 16 and the optical recording medium 7 was set to 135 mm.

On the other hand, the other beam passed through the beam splitter 12was irradiated as a reference beam 5 onto the optical recording medium7. On this occasion, the path of the reference beam 5 was adjusted so asto enable the reference beam to cover the converged region of the signalbeam 4 within the optical recording medium. When the angles of incidenceof the signal beam 4 and the reference beam 5 irradiated onto thesurface of the optical recording medium 7 were measured outside theoptical recording medium 7, the angles of incidence relative to thenormal line of the optical recording medium 7 were found 40 degrees and50 degrees, respectively.

Since the substrate of the optical recording medium 7 was connected withthe external power source (not shown) of 3 kV, an external electricfield of 60 V/μm was applied to the recording layer. When the beams wereirradiated in this manner onto the optical recording medium 7 for 10seconds, a hologram was enabled to be recorded in the optical recordingmedium 7.

Subsequently, the information thus recorded was read as follows. On theoccasion of this reading, the path of the signal beam 4 was shut off bya shutter. Then, the beam which had passed through the beam splitter 12was irradiated, as a reading beam, onto the optical recording medium 7to generate a reconstructed beam. After permitting this reconstructedbeam to pass through a lens 18 (focal length: 150 mm) of the sameconstruction as the lens 16, this reconstructed beam was permitted toenter into a CCD 19 functioning as a reading apparatus. As a result, areconstructed beam having the same intensity distribution as that of thesignal beam 4 was detected. In this case, the lens 18 was disposedperpendicular to the axis of beam and at a position which was spaced 300mm away from the lens 16 so as to enable the axis of beam to align withthe center of the lens 18. The CCD 19 was also disposed perpendicular tothe axis of beam. The distance between the lens 18 and the CCD 19 wasset equal to the focal length of the lens 18.

The information thus recorded was found possible to be reconstructedeven one month later.

EXAMPLE 2

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 1 except that the compound represented by thefollowing chemical formula (12) was employed as a trapping material inplace of the trapping material employed in Example 1.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 3

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 1 except that the compound represented by thefollowing chemical formula (13) was employed as a trapping material inplace of the trapping material employed in Example 1.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 5 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 4

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 1 except that the compound (7-DCST) represented bythe following chemical formula (14) was employed as a non-linear opticalmaterial in place of the non-linear optical material employed in Example1, and the compound represented by the following chemical formula (15)was employed as a trapping material in place of the trapping materialemployed in Example 1.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 5

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 4 except that the compound represented by theaforementioned chemical formula (16) was employed as a trapping materialin place of the trapping material employed in Example 4.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 100° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 6

0.5% by weight of C₇₀, 34.5% by weight of polystyrene (PS), 30% byweight of the compound represented by the aforementioned chemicalformula (5b) as a charge-transport material, 30% by weight of the7-DCST, and 5% by weight of a compound represented by the aforementionedchemical formula (16) as a trapping material were dissolved in a mixtureof THF and toluene to prepare a co-solvent solution.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 7

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 6 except that the compound represented by thefollowing chemical formula (17) was employed as a trapping material inplace of the trapping material employed in Example 6.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 8

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 6 except that the content of the charge-transportmaterial was changed to 25% by weight, that 5% by weight of triphenylamine was added as a plasticizing agent, and that the compoundrepresented by the following chemical formula (18) was employed as atrapping material in place of the trapping material employed in Example6.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 9

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 6 except that the compound represented by thefollowing chemical formula (19) was employed as a trapping material inplace of the trapping material employed in Example 6.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 10

0.5% by weight of C₇₀, 49.5% by weight of the copolymer (y/x=0.05)represented by the aforementioned chemical formula (P1), 7.5% by weightof the EtCz, 7.5% by weight of the BisCzPro, and 35% by weight of the7-DCST were dissolved in a mixture of THF and toluene to prepare aco-solvent solution. The copolymer represented by the chemical formula(P1) was capable of functioning as a charge-transport material and alsoas a trapping material. The content of the copolymer as this trappingmaterial was about 5% by weight.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 11

0.5% by weight of C₇₀, 64.5% by weight of the copolymer (y/x=0.05)represented by the following chemical formula (P2), and 35% by weight ofthe DBMNA were dissolved in a mixture of THF and toluene to prepare aco-solvent solution. The copolymer represented by the chemical formula(P2) was capable of functioning as a charge-transport material, as atrapping material and also as a non-linear optical material. The contentof the copolymer as this trapping material was about 4% by weight.

The polymer represented by the chemical formula (P2) was provided, at aside chain thereof, with the group represented by the aforementionedgeneral formula (A′). More specifically, the polymer contained, asR^(b), a nitrogen-containing heterocyclic group.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 12

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 1 except that the compound represented by thefollowing chemical formula (20) was employed as a trapping material inplace of the trapping material employed in Example 1.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

The compound represented by the chemical formula (20) and employed as atrapping material includes in its molecule a couple of carbazole groupsas an electron-donating group. These carbazole groups are coupled toeach other not through a conjugated system. However, it was confirmedfrom the absorption spectrum of the substance represented by thechemical formula (20) that these carbazole groups were spatiallyoverlapped with each other. Namely, when hole is injected into one ofthese carbazole groups, the electron cloud thereof would be increasinglyoverlapped with the other carbazole group which is being overlappedtherewith.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 1. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

COMPARATIVE EXAMPLE 1

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 4 except that 4-(dimethyl amino)benzalrehyde-diphenylhydrazone (DEH) was employed as a trapping materialin place of the trapping material employed in Example 4.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

When the recording of information was performed under the sameconditions as employed in Example 1, it took 30 seconds for performingthe recording. The reason for this may be attributed to the fact thatsince there was a large difference in ionization potential between thecharge-accepting group of the PVK and the charge-donating group of theDEH, it was more difficult for a hole to be transported from PVK to DEH.In spite of this, the information thus recorded was found impossible tobe reconstructed 30 minutes after the recording. This may be attributedto the fact that since a couple of phenyl groups in the DEH weredirectly bonded to amino group having electron donativity, it wasimpossible to bring about a sufficient change in molecular geometryinside the polymer, so that the retention life of electric charge wasnot sufficiently long.

COMPARATIVE EXAMPLE 2

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 4 except that the compound represented by thefollowing chemical formula (21) was employed as a trapping material inplace of the trapping material employed in Example 4.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was tried under the same conditions as employed in Example1.

As a result, it was impossible to observe a reconstructed beam withinseveral seconds after the termination of the irradiation of recordingbeam. This may be attributed to the fact that in the compoundrepresented by the chemical formula (21) and employed as a trappingmaterial, a couple of carbazole groups were coupled to each other notthrough a conjugated system. Namely, it is assumed that even though thetransport of electric charge from the PVK to the substance representedby the chemical formula (21) might have been quite easy, it wasimpossible to stably retain the electric charge on the surface of thesubstance represented by the chemical formula (21), thereby permittingthe aforementioned phenomenon to generate.

COMPARATIVE EXAMPLE 3

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 4 except that the compound represented by thefollowing chemical formula (22) was employed as a trapping material inplace of the trapping material employed in Example 4. Namely, in thisComparative Example, the compound represented by the chemical formula(22) was employed to function as a trapping material.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm. Further, this recordinglayer was subjected to a poling treatment at a temperature of 80° C.,with both electrodes thereof being connected with a power source of 3kV, to prepare an optical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was tried under the same conditions as employed in Example1.

The compound represented by the chemical formula (22) and employed as atrapping material is similar in structure to the compound represented bythe aforementioned chemical formula (20). However, it was confirmed fromthe absorption spectrum thereof that a couple of carbazole groupstherein were not spatially overlapped with each other. Namely, even ifhole is injected into one of the carbazole groups, it would beimpossible to realize the overlapping of the electron clouds.

When the recording of information was tried under the same conditions asemployed in Example 1 by using this optical recording medium, it wasimpossible to observe a reconstructed beam within several seconds afterthe shut-off of recording beam.

EXAMPLE 13

0.4% by weight of C₇₀, 39.6% by weight of PVK, 15% by weight of EtCz,15% by weight of BisCzPro and 30% by weight of the compound representedby the aforementioned chemical formula (11) were dissolved in a mixtureof THF and toluene to prepare a co-solvent solution. The compoundrepresented by the chemical formula (11) was capable of functioning as atrapping material. Since the content of this compound was 30% by weight,it was possible to bring about the changes of the opticalcharacteristics of this trapping material through the irradiation ofbeam, thereby making it possible to record information.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using a recording apparatus constructed as shown in FIG. 5, therecording and reading hologram were performed as follows. In theoperation of this recording apparatus shown in FIG. 5, the beam emittedfrom a He—Ne laser (output: 30 mW) 10 was at first split into two by abeam splitter 12. One of the beams which was reflected by the beamsplitter 12 was passed through a beam expander 11 to expand the diameterof the beam and then, permitted to pass through a liquid crystal filter15 functioning as an image display element. This liquid crystal filter15 was designed such that the transmissivity thereof was modulated inadvance in conformity with the information to be recorded, and that thetransmitted beam was turned into a signal beam 4, which was thenconverged by a lens 16 (focal length: 150 mm). The distance between thelens 16 and the optical recording medium 7 was set to 135 mm.

On the other hand, the other beam passed through the beam splitter 12was converged by a lens 17 (focal length: 150 mm) to produce a referencebeam 5. In this case, the distance between the lens 17 and the opticalrecording medium was set to 70 mm, and the path of the reference beam 5was adjusted so as to enable the reference beam to cover the convergedregion of the signal beam 4 on the surface of the optical recordingmedium. When the angles of incidence of the signal beam 4 and thereference beam 5 irradiated onto the surface of the optical recordingmedium 7 were measured outside the optical recording medium 7, theangles of incidence relative to the normal line of the optical recordingmedium 7 were found 30 degrees and 60 degrees, respectively.

Since the substrate of the optical recording medium 7 was connected withthe external power source (not shown) of 3 kV, an external electricfield of 60 V/μm was applied to the recording layer. When the beams wereirradiated in this manner onto the optical recording medium 7 for onesecond, a hologram was enabled to be recorded in the optical recordingmedium 7.

Subsequently, the information thus recorded was read as follows. On theoccasion of this reading, the path of the signal beam 4 was shut off bya shutter. Then, the beam which had passed through the beam splitter 12was irradiated, as a reading beam, onto the optical recording medium 7to generate a reconstructed beam. After permitting this reconstructedbeam to pass through a lens 18 (focal length: 150 mm) of the sameconstruction as the lens 16, this reconstructed beam was permitted toenter into a CCD 19 functioning as a reading apparatus. As a result, areconstructed beam having the same intensity distribution as that of thesignal beam 4 was detected. In this case, the lens 18 was disposedperpendicular to the axis of beam and at a position which was spaced 300mm away from the lens 16 so as to enable the axis of beam to align withthe center of the lens 18. The CCD 19 was also disposed perpendicular tothe axis of beam. The distance between the lens 18 and the CCD 19 wasset equal to the focal length of the lens 18.

The information thus recorded was found possible to be reconstructedeven one month later.

EXAMPLE 14

0.3% by weight of C₇₀, 44.7% by weight of PS, 15% by weight of thecompound represented by the aforementioned chemical formula (5a), 5% byweight of triphenyl amine, and 35% by weight of the compound representedby the aforementioned chemical formula (15) were dissolved in a mixtureof THF and toluene to prepare a co-solvent solution. The compoundrepresented by the chemical formula (15) was capable of functioning as atrapping material. Since the content of this compound was 35% by weight,it was possible to bring about the changes of the opticalcharacteristics of this trapping material through the irradiation ofbeam, thereby making it possible to record information.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within 5 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 15

The co-solvent solution obtained in Example 14 was coated on the surfaceof glass plate having no deposition of ITO film by a casting method, andthe resultant layer was treated in the same manner as in Example 1 toform a recording layer having a thickness of 50 μm, thus manufacturingan optical recording medium.

By using this optical recording medium, the recording of information wasperformed under the same conditions as those of Example 13 exceptingthat no external electric field was applied thereon. As a result, it waspossible to record the information even though it took 60 seconds forthe recording.

The information thus recorded was found possible to be reconstructedeven one month later.

EXAMPLE 16

0.3% by weight of C₇₀, 49.7% by weight of PS, 15% by weight of thecompound represented by the aforementioned chemical formula (5a), and35% by weight of the compound represented by the aforementioned chemicalformula (16) were dissolved in a mixture of THF and toluene to prepare aco-solvent solution. The compound represented by the chemical formula(16) was capable of functioning as a trapping material. Since thecontent of this compound was 35% by weight, it was possible to bringabout the changes of the optical characteristics of this trappingmaterial through the irradiation of beam, thereby making it possible torecord information.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using this optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 17

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 14 except that the compound represented by theaforementioned chemical formula (17) was employed as a trapping materialin place of the trapping material employed in Example 14.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within 5 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 18

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 16 except that the compound represented by theaforementioned chemical formula (18) was employed as a trapping materialin place of the trapping material employed in Example 16.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 19

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 16 except that the compound represented by theaforementioned chemical formula (19) was employed as a trapping materialin place of the trapping material employed in Example 16.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within one second. The information thus recorded was foundpossible to be reconstructed even one month later.

EXAMPLE 20

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 13 except that the compound represented by theaforementioned chemical formula (20) was employed as a trapping materialin place of the trapping material employed in Example 13.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

By using the optical recording medium thus obtained, the recording ofinformation was performed under the same conditions as employed inExample 13. As a result, it was found possible to sufficiently achievethe recording within 10 seconds. The information thus recorded was foundpossible to be reconstructed even one month later.

The reason for this can be attributed to the fact that, as explainedwith reference to Example 12, the compound represented by the chemicalformula (20) was capable of functioning as a trapping material.

EXAMPLE 21

By using the optical recording medium manufactured in Example 5, therecording of information was performed under the same conditions asemployed in Example 13. As a result, it was found possible tosufficiently achieve the recording within 10 seconds. The informationthus recorded was found possible to be reconstructed even one monthlater.

EXAMPLE 22

By using the optical recording medium manufactured in Example 11, therecording of information was performed under the same conditions asemployed in Example 13. As a result, it was found possible tosufficiently achieve the recording within 10 seconds. The informationthus recorded was found possible to be reconstructed even one monthlater.

COMPARATIVE EXAMPLE 4

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 13 except that DEH was employed as a trappingmaterial in place of the trapping material employed in Example 13.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

When the recording of information was tried by using the opticalrecording medium under the same conditions as employed in Example 13, itwas impossible to perform the recording. In the case of the DEH, acouple of phenyl groups, relatively large in structure, were directlybonded to amino group having electron donativity. Therefore, it wasimpossible to bring about a sufficient change in molecular geometryinside the polymer, so that the structural change on the occasion whenelectric charge was retained therein was relatively small, thus makingit impossible to bring about a sufficient change in the opticalcharacteristics of the recording layer.

COMPARATIVE EXAMPLE 5

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 13 except that the compound represented by theaforementioned chemical formula (21) was employed as a trapping materialin place of the trapping material employed in Example 13.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

When the recording of information was tried by using the opticalrecording medium under the same conditions as employed in Example 13, itwas impossible to perform the recording. In the case of the compoundrepresented by the aforementioned chemical formula (21), a couple ofcarbazole groups having electron-donativity were coupled with each othernot through a conjugated system. Therefore, it was impossible to bringabout a sufficient change in the optical characteristics of therecording layer on the occasion when electric charge was retainedtherein.

COMPARATIVE EXAMPLE 6

A co-solvent solution was prepared by repeating the same procedures asset forth in Example 13 except that the compound represented by theaforementioned chemical formula (22) was employed as a trapping materialin place of the trapping material employed in Example 13.

Then, by the same procedures as set forth in Example 1, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 50 μm, thus manufacturing anoptical recording medium.

When the recording of information was tried by using the opticalrecording medium under the same conditions as employed in Example 13, itwas impossible to perform the recording. In the case of the compoundrepresented by the aforementioned chemical formula (22), a couple ofcarbazole groups having electron-donativity were incapable ofinteracting with each other.

EXAMPLE 23

The co-solvent solution prepared in the aforementioned Example 1 wasdropped and coated on the surface of glass substrate by a casting methodto form a recording layer, thus obtaining the optical recording medium.Glass substrates used in this Example were not ITO deposited. On thisoccasion, the thickness of this recording layer was adjusted to 250 μmby using a Teflon (registered trademark) spacer. Further, the glassplate disposed on the objective lens side was quenched to remove theglass plate, thus obtaining the optical recording medium.

By using a recording apparatus constructed as shown in FIG. 7, therecording and reading data were performed as follows. In the operationof this recording apparatus shown in FIG. 7, the output beam emittedfrom a semiconductor laser was shaped into a parallel beam by acollimator lens and then, converged in the optical recording medium byan objective lens. In this case, in order to enable the beam to convergeat any desired position in the depth-wise direction of the recordinglayer, a lens (NIKON Co., Ltd., tradename: CF IC LCD Plan CR,magnification: 100 times), which is long in operating distance and largein NA (Numerical Aperture), was employed.

First of all, an injection current to the semiconductor laser wasminimized and the intensity of irradiating beam was sufficientlylowered, under which conditions the position where the semiconductorlaser was intended to be converged was aligned with the surface of therecording layer. Subsequently, under the condition where the irradiationof beam was stopped, the stage was moved so as to enable the beam tofocus at a position spaced 10 μm away from the surface of the recordinglayer.

Thereafter, a recording beam was irradiated by enlarging the injectioncurrent for a predetermined period of time. Further, in order to performthe recording of information at a different position in depth-wise ofthe recording layer, the stage was moved, while stopping the irradiationof beam, so as to move the focal point to a position which was 30 μmdeeper than the previous position. In order to record information atthis position, the injection current was modulated for irradiating arecording beam. These procedures were repeated to record four data atdifferent depths within the recording layer. As described above, byusing the stage and while stopping the irradiation of beam, the focalpoint is moved to a desired region before performing the irradiation ofa recording beam for a predetermined period of time, thereby making itpossible to record information at a desired region within the recordinglayer.

Thereafter, by using the stage and while stopping the irradiation ofbeam, the optical recording medium was moved so as to enable the axis ofbeam to pass through the center of the recording region and also enablethe focal point to be positioned at a region which was 5 μm deeper thanthe previous recorded region. Then, the injection current was modulatedso as to make the quantity of beam smaller than the recording beam by amagnitude of about 1/100, and the intensity, at this region, oftransmitted beam “Is” was measured with a detector lying on the opticalaxis of the beam. Then, the ratio between this intensity of beam “I_(s)”and the intensity of beam “I_(r)” that had been measured in advance byirradiating the same quantity of beam into the recording layer wasdetermined. As a result, the ratio I_(s)/I_(r) was 1.2. On the otherhand, when the position of focus was sufficiently spaced away from therecording region, the ratio I_(s)/I_(r) was 1.0.

EXAMPLE 24

0.5% by weight of C₇₀, 34.5% by weight of PS, 30% by weight of thecharge-transport material represented by the aforementioned chemicalformula (5b), 30% by weight of DBMNA, and 5% by weight of the compoundrepresented by the aforementioned chemical formula (16) as a trappingmaterial were dissolved in a mixture of THF and toluene to prepare aco-solvent solution.

Then, by the same procedures as set forth in Example 23, the co-solventsolution thus obtained was coated on the surface of substrate to form arecording layer having a thickness of 250 μm, thus manufacturing anoptical recording medium. By using the optical recording medium thusobtained, the irradiation of beam for a period of one second wasrepeated in the same manner as in Example 23, thereby sequentiallyobtaining four data. Then the ratio I_(s)/I_(r) was found to be 1.3.

EXAMPLE 25

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Example 5 was coated on the surface of substrate toform a recording layer having a thickness of 250 μm, thus manufacturingan optical recording medium. By using the optical recording medium thusobtained, the irradiation of beam for a period of one second wasrepeated in the same manner as in Example 23, thereby sequentiallyobtaining four data. Then the ratio I_(s)/I_(r) was found to be 1.4.

EXAMPLE 26

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Example 11 was coated on the surface of substrateto form a recording layer having a thickness of 250 μm, thusmanufacturing an optical recording medium. By using the opticalrecording medium thus obtained, the irradiation of beam for a period ofone second was repeated in the same manner as in Example 23, therebysequentially obtaining four data. Then the ratio I_(s)/I_(r) was foundto be 1.2.

EXAMPLE 27

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Example 16 was coated on the surface of substrateto form a recording layer having a thickness of 250 μm, thusmanufacturing an optical recording medium. By using the opticalrecording medium thus obtained, the irradiation of beam for a period ofone second was repeated in the same manner as in Example 23, therebysequentially obtaining four data. Then the ratio I_(s)/I_(r) was foundto be 1.3.

COMPARATIVE EXAMPLE 7

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Comparative Example 4 was coated on the surface ofsubstrate to form a recording layer having a thickness of 250 μm, thusmanufacturing an optical recording medium. By using the opticalrecording medium thus obtained, the irradiation of beam for a period ofone second was repeated in the same manner as in Example 23, therebysequentially obtaining four data. Then the ratio I_(s)/I_(r) was foundto be approximately 1.

COMPARATIVE EXAMPLE 8

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Comparative Example 5 was coated on the surface ofsubstrate to form a recording layer having a thickness of 250 μm, thusmanufacturing an optical recording medium. By using the opticalrecording medium thus obtained, the irradiation of beam for a period ofone second was repeated in the same manner as in Example 23, therebysequentially obtaining four data. Then the ratio I_(s)/I_(r) was foundto be approximately 1.

COMPARATIVE EXAMPLE 9

By the same procedures as set forth in Example 23, the co-solventsolution prepared in Comparative Example 6 was coated on the surface ofsubstrate to form a recording layer having a thickness of 250 μm, thusmanufacturing an optical recording medium. By using the opticalrecording medium thus obtained, the irradiation of beam for a period ofone second was repeated in the same manner as in Example 23, therebysequentially obtaining four data. Then the ratio I_(s)/I_(r) was foundto be approximately 1.

As explained above, it is possible, according to the present invention,to provide an optical recording medium wherein information is enabled tobe recorded as a hologram, and the time required for the recording canbe shortened while ensuring a practical recording life. Further, it ispossible, according to the present invention, to provide an opticalrecording apparatus which is designed to record information through suchan optical recording medium.

The optical recording medium according to the present invention isuseful in realizing a high-density recording and hence is very valuablein industrial viewpoint.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical recording medium comprising a recording layer containing acharge-generating material capable of generating an electron and a holeby light irradiation, a charge-transport material enabling at least saidhole to be transported to isolate said electron and said hole, and atrapping material retaining said hole, the optical characteristics ofsaid recording layer being changed in accordance with changes in spatialdistribution of said electron and said hole, and said trapping materialbeing a polymer having, at a side chain thereof, a group represented bythe following general formaula (A′):

wherein CB1 is a conjugated system; and R^(a) and R^(b) may be the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group selected from the followinggroups and bonded through an unsaturated carbon atom of saidheterocyclic group to said conjugated system:

and wherein a residual group having electron donativity and being otherthan said nitrogen-containing heterocyclic group is at least oneselected from the group consisting of allyl alkane; nitrogen-containingcyclic compound; oxygen-containing compound; sulfur-containing compound;and the following groups:

wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.
 2. The optical recording mediumaccording to claim 1, wherein at least one of R^(a) and R^(b) is a grouprepresented by the formula (1c) or (1h).
 3. An optical recording mediumcomprising a recording layer containing a charge-generating materialcapable of generating a hole and an electron by light irradiation, acharge-transport material enabling at least said hole to be transportedto isolate said hole and said electron, and a trapping materialretaining said hole, the optical characteristics of said recording layerbeing changed in accordance with changes in spatial distribution of saidhole and said electron, and said trapping material being a polymerhaving, at a side chain thereof, a group represented by the followinggeneral formula (B′):

wherein CB1 is a conjugated system; and R^(c) is a monovalent orbivalent, nitrogen-containing heterocyclic group selected from thegroups shown below and bonded through an unsaturated carbon atom of saidheterocyclic group to said conjugated system:


4. The optical recording medium according to claim 3, wherein R^(c) is agroup represented by the formula (1c) or (1h).
 5. An optical recordingapparatus comprising: a light source emitting a beam; a beam splitterseparating said beam into two beams; a first optical device which isconfigured to provide one of these separated beams with information tobe recorded; an optical recording medium comprising a recording layercontaining a charge-generating material capable of generating anelectron and a hole by light irradiation, a charge-transport materialenabling at least said hole to be transported to isolate said electronand said hole, and a trapping material retaining said hole, the opticalcharacteristics of said recording layer being changed in accordance withchanges in spatial distribution of said electron and said hole; and asecond optical device which is configured to directing said separatedbeams so as to intersect each other within said recording medium, theintersecting beams making interference fringes within said recordinglayer of said optical recording medium to write information; whereinsaid trapping material is a polymer having, at a side chain thereof, agroup (A′):

wherein CB1 is a conjugated system; and R^(a) and R^(b) may be the samewith or different from each other and are individually a group havingelectron donativity, at least one of R^(a) and R^(b) being anitrogen-containing heterocyclic group selected from the followinggroups and bonded through an unsaturated carbon atom of saidheterocyclic group to said conjugated system:

and wherein a residual group having electron donativity and being otherthan said nitrogen-containing heterocyclic group is at least oneselected from the group consisting of allyl alkane; nitrogen-containingcyclic compound; oxygen-containing compound; sulfur-containing compound;and the following groups:

wherein R¹ and R² may be the same with or different from each other andare individually hydrogen atom, alkyl group, alkoxy group, phenyl group,naphthyl group, tolyl group, benzyl group, benzothiazole group,benzoxazolyl group, benzopyrrol group, benzoimidazolyl group,naphthothiazolyl group, naphthoxazolyl group, naphthopyrrol group,naphthoimidazolyl group or hydroxyl group; at least one of R¹ and R²being hydrogen atom, or a group to be bonded, through oxygen atom orsaturated carbon atom, to nitrogen atom.
 6. An optical recordingapparatus comprising: a light source emitting a beam; a beam splitterseparating said beam into two beams; a first optical device which isconfigured to provide one of these separated beams with information tobe recorded; an optical recording medium comprising a recording layercontaining a charge-generating material capable of generating anelectron and a hole by light irradiation, a charge-transport materialenabling at least said hole to be transported to isolate said electronand said hole, and a trapping material retaining said hole, the opticalcharacteristics of said recording layer being changed in accordance withchanges in spatial distribution of said electron and said hole; and asecond optical device which is configured to directing said separatedbeams so as to intersect each other within said recording medium, theintersecting beams making interference fringes within said recordinglayer of said optical recording medium to write information; whereinsaid trapping material is a polymer having, at a side chain thereof, agroup represented by the following general formula (B′):

wherein CB1 is a conjugated system; and R^(c) is a monovalent orbivalent, nitrogen-containing heterocyclic group selected from thegroups shown below and bonded through an unsaturated carbon atom of saidheterocyclic group to said conjugated system: